Newbie question about "0dB" and "VU" meter

Introduce yourself! Recommended for people who are just starting out, as opposed to experienced lathe cutters who are new members.

Moderators: piaptk, tragwag, Steve E., Aussie0zborn

User avatar
lucien
Posts: 55
Joined: Wed May 30, 2012 1:01 pm
Location: France

Newbie question about "0dB" and "VU" meter

Post: # 28217Unread post lucien
Tue Dec 24, 2013 8:31 pm

Hello everyone,

I don't have a lathe so I lack hands on experience, hence the newb question. I did a search before and find this link https://www.lathetrolls.com/viewtopic.php?f=1&t=3548&p=21470 but I'm still missing very basic things.

Regarding "Vu meters", is this referring to mechanical VU meters with a -20 +3 range?
I guess the typical 0 ref 5cm/s 1Khz tone is a nominal level, right?
How many dB can reach loud cuts? Generally speaking, for those cutting on a regular basis what's the typical cutting level?

For those using a T-560 vinyl recorder with a LED "VU-meter" with a -46 + 12 range, to which value are you calibrating "0dB"? 1,23 or 0,77V?

I'm still at the level of trying to compare pressed records level. I have a turntable with a line level output. With my cartridge when I play a 0 dB reference 1khz tone I have an output value of approximately 0.250V. I had to calibrate my VU meters to 0,77V otherwise it was always in the red when I was playing records with music. I finally got lost in the world of dB and velocity.

Lucien

User avatar
markrob
Posts: 1623
Joined: Wed Nov 28, 2007 1:14 am
Location: Philadelphia Area

Re: Newbie question about "0dB" and "VU" meter

Post: # 28220Unread post markrob
Wed Dec 25, 2013 12:24 pm

Hi,

If its a real mechanical VU meter and you have a test record with a 5cm/sec 1Khz reference tone, then setting you meter to read 0 when this is played back is a good starting point. If you play back really hot commercial pressings with this as a reference, you may indeed find that they end up in the red. However, this may be a good level to start out cutting with until you gain experience with your cutter head. If the meter is a fast responding or peak reading LED type (not averaging), I'd calibrate this reference tone to -10 to -12db bleow 0db. This allows for short term peaks that a VU meter will not see. Once you establish a playback reference level, you'll need to calibrate this with your cutting signal chain. To do that, start out by driving the head with the AC voltage needed to achieve drive 1 watt at 1Khz and cut this tone. Now playback your cut and see where is sits in relation to the playback calibration. Adjust you record reference up or down based on this result. For example if your 1 watt cut reads -2db when you playback you will need to increase your drive level from 1 watt to about 1.6 watts (+2db). This equates to about 26% more drive voltage.

Mark

User avatar
Steve E.
Site Admin
Posts: 1914
Joined: Fri Jun 24, 2005 3:24 pm
Location: Brooklyn, New York, USA
Contact:

Re: Newbie question about "0dB" and "VU" meter

Post: # 28228Unread post Steve E.
Thu Dec 26, 2013 2:35 pm

markrob wrote:Hi,

If its a real mechanical VU meter and you have a test record with a 5cm/sec 1Khz reference tone,
Mark, what is being measured when we see "5cm/sec"? This confuses me, perpetually.

User avatar
opcode66
Posts: 2700
Joined: Sun Nov 08, 2009 10:56 pm
Contact:

Re: Newbie question about "0dB" and "VU" meter

Post: # 28230Unread post opcode66
Thu Dec 26, 2013 3:15 pm

The velocity if the cutting/playback stylus at the center of each excusion. At the far end off each excision the stylus velocity is 0 cm/s and the direction of motion flip flops. You can also calculate acceleration of the stylus in cm/s/s. Remember, its all about tge excusions of the groove. Volume is physically traced as amplitude or amount of excusion. For a 1k tone, the excursions will produce movement of the stylus at 5 cm/s when capturing 0vu tone ona properly calibrating system. 0 VU = +4dbm = 1.228 Volts AC electrically measured. When physically measured = 5cm/s for 1k tone at 0vu. Decibels have absolutely no meaning in relation to volume when recording to disc. Only stylus velocity does.
Cutting, Inventing & Innovating
Groove Graphics, VMS Halfnuts, MIDI Automation, Professional Stereo Feedback Cutterheads, and Pesto 1-D Cutterhead Clones
Cutterhead Repair: Recoiling, Cleaning, Cloning of Screws, Dampers & More
http://mantra.audio

User avatar
markrob
Posts: 1623
Joined: Wed Nov 28, 2007 1:14 am
Location: Philadelphia Area

Re: Newbie question about "0dB" and "VU" meter

Post: # 28232Unread post markrob
Thu Dec 26, 2013 7:17 pm

opcode66 wrote:The velocity if the cutting/playback stylus at the center of each excusion. At the far end off each excision the stylus velocity is 0 cm/s and the direction of motion flip flops. You can also calculate acceleration of the stylus in cm/s/s. Remember, its all about tge excusions of the groove. Volume is physically traced as amplitude or amount of excusion. For a 1k tone, the excursions will produce movement of the stylus at 5 cm/s when capturing 0vu tone ona properly calibrating system. 0 VU = +4dbm = 1.228 Volts AC electrically measured. When physically measured = 5cm/s for 1k tone at 0vu. Decibals have absolutely no meaning in relation to volume when recording to disc. Only stylus velocity does.

Hi,

db most certainly does equate to loudness when cutting. That's what the db scale was originally invented to track. The scale is logarithmic due to the fact that our hearing responds roughly in that way. As designed, each 10db increase in level results in an approximate perceived doubling of loudness. 3db represents a doubling of power and 6db a doubling of voltage.

At a single frequency and reference level (e.g. 1Khz at 5cm/s), a 10db increase in cutting level would result in a 15.8cm/sec recorded velocity and when played back would be heard about twice as loud as the same tone cut at 5cm/s.

When you cut at a constant velocity vs. frequency, the peak to peak excursion of the cutting stylus increases as the frequency decreases. You can calculate the peak excursion by:

S=V/(2 x pi x F)

Where S is the peak excursion. F is the frequency in hz, and V is the peak velocity

So in the case of a 1Khz 5cm/sec cut, the excursion is 7.95 micrometers. The same velocity cut at 20hz would result in a peak excursion of 397 micrometers. A much larger excursion. That's the reason that the RIAA curve was designed. At low frequencies, we cut at a constant excursion (amplitude) vs. frequency. Otherwise, the groove to groove distance would need to be very large. Once we get up to higher frequencies, the cutting stylus velocity becomes too high to cut without issue, so the RIAA curve transitions to a constant velocity mode. At still higher frequencies, the curve transitions back to constant amplitude since the excursions have now decreased in this region of the curve. This keeps the high frequencies from getting swallowed up by noise.

As far as 0dbm goes, that is an established standard reference level. However, you can use any reference as needed to calibrate your system. When a db value is provided it is meaningless without stating the reference point. You can find dbm, dbV, dbVu, dbFS, and many others tossed around. These all have various references and have uses defined uses. But you can establish your own reference level to aid in setting an appropriate cutting level. So, if you set your meter to read 0db when you cut a 5cm/s 1Khz tone, you have established a reference point. If that turns out to be 5.123Vac RMS across your cutting head or 1.56Vac RMS at the input to your power amp, so be it.

Mark

Mark

User avatar
opcode66
Posts: 2700
Joined: Sun Nov 08, 2009 10:56 pm
Contact:

Re: Newbie question about "0dB" and "VU" meter

Post: # 28236Unread post opcode66
Fri Dec 27, 2013 1:31 pm

Cant measure db's on a record. You can measure stylus excursion and velocity. That's all I was saying.
Cutting, Inventing & Innovating
Groove Graphics, VMS Halfnuts, MIDI Automation, Professional Stereo Feedback Cutterheads, and Pesto 1-D Cutterhead Clones
Cutterhead Repair: Recoiling, Cleaning, Cloning of Screws, Dampers & More
http://mantra.audio

User avatar
flozki
Posts: 571
Joined: Fri May 19, 2006 4:40 am
Location: switzerland
Contact:

Re: Newbie question about "0dB" and "VU" meter

Post: # 28241Unread post flozki
Fri Dec 27, 2013 6:24 pm

get a testrecord.

the 7.95 um is a little bit difficult to measure without good tools.

there are expensive ones (cardas or "Ultimate Analogue Test LP" and cheap ones e.g. vinylike 7inch.
most of the expensive ones are quite off. at least 0.5-1 dB.
not sure about the Ultimate Analogue Test LP. i can say that left right is quite accurate.
i only played back that one on a playback system and compared with other test records.
I have not measured light beam yet, because i dont own that record.
but i try to measure every record with light beam and update my list.
the vinylike 7inch is so far one of the most accurate. -.28db/+0.37db on the final pressing.
but maybe there are some better ones. i still miss a lot of those records.

but they are all within 1 db. so i guess its good enough for a non feedback cutterhead.
then play that record with your regular playback system and read the signal on your meter. thats your reference level.
then cut your 1k tone and read this one. thats the level you can go. but also you should read thd. for non feedback head that will be much higher.

another simple method check datasheet of your playback system.
they tell you typical level for 5cm/s
ortofon red is 5.5mV for 5cm/s@1kHz. if you have equipment to read that. you can also use this value as reference levels.

another more accurate test is light beam measurment.
for a good measurment you need a special measurment tool.
but you can also do with a torch and a ruler. not to best resolution but within 1 db = thats approx 4mm in difference.
for 5cm/s and 33rpm you should read 40.8mm NAB
for 8cm/s and 33rpm you should read 64.82mm DIN Norm

and then there is the main limit of playback such a signal.
strongly depends on the pickup (plus speed / diameter) you have. with a horrible concorde you are not able to track a 8cm/s without distortion.
so make all tests on 5cm/s reference levels and thats why all cartridges are referenced to 5cm/s
if you go +4dB from that (thats equal 8cm/s) you adefinitely need well adjusted tonearm and decent pickup.
so it does not make sense to go much higher than +4 /+6 on such a system.

happy bricolage.f.

User avatar
emidisc
Posts: 220
Joined: Mon Jun 07, 2010 7:57 pm
Location: lancashire

Re: Newbie question about "0dB" and "VU" meter

Post: # 28254Unread post emidisc
Sun Dec 29, 2013 1:37 pm

This is something I really need to do to my set up I just wish I understood the process?
I have the vinylike 7" test tone record but then I'm lost I'm afraid!
can anybody suggest some reading that could get me up to speed on this perhaps?

Emidisc

User avatar
Steve E.
Site Admin
Posts: 1914
Joined: Fri Jun 24, 2005 3:24 pm
Location: Brooklyn, New York, USA
Contact:

Re: Newbie question about "0dB" and "VU" meter

Post: # 28329Unread post Steve E.
Fri Jan 03, 2014 1:41 pm

I still don't understand how anyone is actually measuring that velocity. What machine does that?

Or, are these just the standards, and we are reading them off records, comparing them?

I'm assuming the latter. But,again, if so, how the heck did someone measure that? Forgive me if this is a tangent. I am finding this thread extremely useful.

User avatar
opcode66
Posts: 2700
Joined: Sun Nov 08, 2009 10:56 pm
Contact:

Re: Newbie question about "0dB" and "VU" meter

Post: # 28335Unread post opcode66
Fri Jan 03, 2014 5:09 pm

There is no specific tool I'm aware of to do this. Except possibly the laser based playback system. It likely could maintain an ongoing average velocity calculation.

Flozki mentions the light beam test. That is the standard way to actually measure this on disc.
Cutting, Inventing & Innovating
Groove Graphics, VMS Halfnuts, MIDI Automation, Professional Stereo Feedback Cutterheads, and Pesto 1-D Cutterhead Clones
Cutterhead Repair: Recoiling, Cleaning, Cloning of Screws, Dampers & More
http://mantra.audio

User avatar
markrob
Posts: 1623
Joined: Wed Nov 28, 2007 1:14 am
Location: Philadelphia Area

Re: Newbie question about "0dB" and "VU" meter

Post: # 28341Unread post markrob
Sat Jan 04, 2014 7:00 am

Steve E. wrote:I still don't understand how anyone is actually measuring that velocity. What machine does that?

Or, are these just the standards, and we are reading them off records, comparing them?

I'm assuming the latter. But,again, if so, how the heck did someone measure that? Forgive me if this is a tangent. I am finding this thread extremely useful.
Hi,

The magnetic pickup you use to playback you records is a velocity measuring device. If you use the output of the pickup directly (no RIAA curve), the output voltage vs. time is the recorded velocity. That's why the output voltage of a magnetic pickup is given in the form of mv/cm/sec. The accuracy of the measurement is subject to the quality of the pickup used. This becomes an issue at higher frequencies. The optical light band method is more accurate. You can also calculate velocity if you measure the excursion and frequency and apply the formula I gave in my earlier post.

I suspect that velocity was used as the measurement standard for disk recording because of the fact that early electrical pickups were velocity responsive. Crystal and ceramic pickups came later and these are amplitude (excursion) responding devices. Since position, velocity, and acceleration are all interrelated, any can and could be used as a measurement standard.

Mark

User avatar
lucien
Posts: 55
Joined: Wed May 30, 2012 1:01 pm
Location: France

Re: Newbie question about "0dB" and "VU" meter

Post: # 32675Unread post lucien
Sat Dec 13, 2014 8:21 am

Hello everyone,

I've been away for a year so sorry for the late reply.
Thanks for your answers. Will study that in deep :)

Lucien

User avatar
lucien
Posts: 55
Joined: Wed May 30, 2012 1:01 pm
Location: France

Re: Newbie question about "0dB" and "VU" meter

Post: # 32929Unread post lucien
Sat Dec 27, 2014 2:14 pm

flozki> there is a list of test discs (not only frequency) on page 45 of this paper : http://sts.kahaku.go.jp/diversity/document/system/pdf/083.pdf

The last three ones are Toshiba EMI, Nippon Victor and Nippon Columbia


I've started to read this paper. It seems excellent. It's a pity there is only one page in English.
Despite the language barrier, I think it's still worth to have a look at all the pictures :)
Will probably be a source of inspiration for my further "Japanese" investigations.

User avatar
Steve E.
Site Admin
Posts: 1914
Joined: Fri Jun 24, 2005 3:24 pm
Location: Brooklyn, New York, USA
Contact:

Re: Newbie question about "0dB" and "VU" meter

Post: # 32936Unread post Steve E.
Sun Dec 28, 2014 3:20 pm

This is a fantastic, essential thread. I am sticky-ing it and I may move it to another section.

User avatar
gold
Posts: 365
Joined: Tue Apr 28, 2009 12:03 am
Location: Brooklyn

Re: Newbie question about "0dB" and "VU" meter

Post: # 32938Unread post gold
Sun Dec 28, 2014 3:58 pm

opcode66 wrote:Cant measure db's on a record. You can measure stylus excursion and velocity. That's all I was saying.
dB is just a relative scale. It has to be tied to a reference to have any meaning. It doesn't matter if it's a record or a tape or a wire recorder. The reference level for a record is 7cm/sec stereo lateral. The original NAB standard for magnetic tape was 185 nW/M. No one wants to deal with units that don't translate to the outside world. Hence dBref. Typically the reference level for a pro studio is +4dBu. 0.775vac = 0dBu. +4dBu=1.228vac.

The only way to directly measure level on a disc is via the light beam method or via calibrated microscope.

User avatar
mossboss
Posts: 2050
Joined: Sun Jul 01, 2007 8:18 am
Location: Australia.

Re: Newbie question about "0dB" and "VU" meter

Post: # 33013Unread post mossboss
Sat Jan 03, 2015 7:03 pm

Well it is a good thread indeed
For any one who can get hold of the VMS manual and take a quick look at the literature included in the manual, about the Acceleration limiter BSB 74, will make it quite clear
I have known people who called this just a limiter often taking it off line
It is not, it really works protecting the cutting head, while keeping as much real estate as possible to place music on the lacquer rather than empty land area
A good read about the RIAA curve, another interesting innovation, will help in understanding even though Mr Rob has done an excellent job of it
Cheers
Chris

User avatar
Stevie342000
Posts: 497
Joined: Mon Dec 21, 2009 2:12 pm

Re: Newbie question about "0dB" and "VU" meter

Post: # 33014Unread post Stevie342000
Sun Jan 04, 2015 4:04 am

gold wrote:
opcode66 wrote:Cant measure db's on a record. You can measure stylus excursion and velocity. That's all I was saying.
dB is just a relative scale. It has to be tied to a reference to have any meaning. It doesn't matter if it's a record or a tape or a wire recorder. The reference level for a record is 7cm/sec stereo lateral. The original NAB standard for magnetic tape was 185 nW/M. No one wants to deal with units that don't translate to the outside world. Hence dBref. Typically the reference level for a pro studio is +4dBu. 0.775vac = 0dBu. +4dBu=1.228vac.

The only way to directly measure level on a disc is via the light beam method or via calibrated microscope.

With Reference to the Light Beam Method - Buchmann-Meyer there is an article on the BBC Research Papers Website on how to set this up - it looks cumbersome but there must be a more modern way of doing it. It is complicated but I can just about figure it out, keep re-reading the paper until it sinks in.

I can tell you it is Research Paper C077 1950/20 which is about page 22, it should be found here: http://www.bbc.co.uk/rd/search?Type=Publications&Decade=1950&page=22 or if you want to start at the beginning here: http://www.bbc.co.uk/rd/publications or if you wish to more research then you can start here: http://bbceng.info/index.htm

You can get lost in here for days, trust me I've been there, the technical instructions are interesting but there is lot more information on the background research that the BBC did in all areas of broadcast and recording. It's one of the few places and organisations where you will find information on disc cutting and recording. There is a PDF of more research papers than is listed on the first link, it should be in the reference section - try a search.

The simplest way to set your cutting level on the head is to measure output from a known source (reference disc cut at X cm/s) at 1kHz, then to cut a disc and measure the output. Starting off with Level going into your cutting amp at a conservative setting (you do not want to burn out your cutting head) - a 500 mA fuse in the cutter head line as well as VU or PPM meter should help - start at -10dB - but you have to set your replay levels and leave them set - then set your gain slowly increasing and doing a further cut until you reach the same operating level from cutting as the reference disc.

There is a trade off in cutting levels and that is time but the length of your material you wish to cut will also determine your cutter level. Its a complicated but simple process, no wonder it was referred to as a dark art. Like most things it is a series of compromises. Once you have cut a few test discs and then measured the output on replay you should be able to make a note of the reference level they were cut at on your VU meter prior to cutter - so you can set the level pretty much every time. A lot of this will done via pre-sets or automatically on a Neumann or Scully - a Rek-o-Kut or Presto will have a reference level on the meter.

With time judgement and experience you should be able to set your cutter to whatever reference level you wish - determined by the length in time of the material you wish to cut. You actually do not need that much power in watts to drive a cutter to a known acceleration - even Neumanns on have 10 watt coils (correct me if I am wrong) is just an example. But your cutter head drive amp may be anywhere from 15 to 600 watts. Its the instantaneous peak power your interested in - it has to be clean and unclipped.

Cutting discs is a whole series of compromises which must all come together with the set parameters in order to achieve the desired outcome - simples. Under no circumstances should you operate your cutter head without a fuse - you will or may have to determine the size of the fuse but start of with 500mA - remember if you have a 500 watt amp and 10 watt cutter what will happen if you get a surge from a switching click to your cutter head?

Don't forget that when you set your reference level at 1kHz e.g = 7cm/s to take into account the Inverse RIAA boost above 1kHz - there is limit - you could push it to 20cm/s at 1kHz but then track-ability becomes an issue on replay.

Remember a scale is only a measure and that you have a number of operating levels - it is simple once you get your head around the whole process - mistakes will be made they happen - but learn from them.

Yes interesting thread - I hope I have not confused the issue - it is complicated but at the same it is simple as long as you stick to the rules. This is where books such as the AES papers and the Boden book come in - it helps you to understand the basic principles and some not so basic in the case of the AES papers. Which I must get a copy of......Well off I must go now to read about measuring light patterns again.

User avatar
Stevie342000
Posts: 497
Joined: Mon Dec 21, 2009 2:12 pm

Re: Newbie question about "0dB" and "VU" meter

Post: # 33015Unread post Stevie342000
Sun Jan 04, 2015 4:35 am

The objectives of setting your cutting level to a known reference is or maybe:

1. to obtain a flat response from 20 Hz to 20kHz at a known cutting level - without blowing your cutter head or compromising the groove cut.
2. to cut at a known reference level
3. to determine the frequency response of your replay cartridge and cutting chain and to fulfill objective 1.
4. to set a cutting level based on the time and groove pitch (assuming your not using a lathe with automatic groove pitch).
5. Time determines your groove pitch and cutting level to a known reference.

You should measure the output from the cartridge should be done with RIAA - as you may wish to compensate for the response of the cartridge - so that it is flat when RIAA is applied - you may not do this but you should measure it as it is your reference starting point - make a note of the level obtained on a VU meter at each Frequency or just at 1Khz reference.

Cut your disc (noting level on the VU meter in the cutting chain (after power amp prior to cutter head) - replay - measure response to former reference taken from the Test Tone reference disc at x y or z reference level.

You can cut a number of frequencies for which a reference level was taken at the same settings from the replay of a disc cut a a known reference level - which will give you the response of your cutter head - remembering not to cut with Inverse RIAA in circuit - you can then add compensation to your cutter chain (in either the digital or analogue domain) to ensure that the cutter is cutting flat. This is partly the concept of Feedback cutter heads.

Once you have determined what the reference level is you can set it every time by referring back to your chart having done a number of tests at x y or z levels - taken from the readings on your VU meter prior to cutting head.

User avatar
opcode66
Posts: 2700
Joined: Sun Nov 08, 2009 10:56 pm
Contact:

Re: Newbie question about "0dB" and "VU" meter

Post: # 33028Unread post opcode66
Mon Jan 05, 2015 4:51 am

gold wrote:
opcode66 wrote:Cant measure db's on a record. You can measure stylus excursion and velocity. That's all I was saying.
dB is just a relative scale. It has to be tied to a reference to have any meaning. It doesn't matter if it's a record or a tape or a wire recorder. The reference level for a record is 7cm/sec stereo lateral. The original NAB standard for magnetic tape was 185 nW/M. No one wants to deal with units that don't translate to the outside world. Hence dBref. Typically the reference level for a pro studio is +4dBu. 0.775vac = 0dBu. +4dBu=1.228vac.

The only way to directly measure level on a disc is via the light beam method or via calibrated microscope.
You quote me. Go down a rabbit hole. Then say exactly the same thing I said in different words. Since we can't directly measure stylus velocity, the only way to readily measure it is with the light beam method. This is why I was so keen to acquire a buchman myer light that actually works. Eventually, I will be using it to verify test cuts with my cutterhead project.
Cutting, Inventing & Innovating
Groove Graphics, VMS Halfnuts, MIDI Automation, Professional Stereo Feedback Cutterheads, and Pesto 1-D Cutterhead Clones
Cutterhead Repair: Recoiling, Cleaning, Cloning of Screws, Dampers & More
http://mantra.audio

User avatar
lucien
Posts: 55
Joined: Wed May 30, 2012 1:01 pm
Location: France

Re: Newbie question about "0dB" and "VU" meter

Post: # 33031Unread post lucien
Mon Jan 05, 2015 10:17 am

Yes interesting thread - I hope I have not confused the issue - it is complicated but at the same it is simple as long as you stick to the rules.
No your explanations are clear :)
Thank you

It's quite difficult in theory but it seems easier in practice thanks to the step by step procedure and your advice.


Otherwise, I'm just wondering, when talking about test cuts and test records, what could be the "amount of error", if any, introduced by the record pressing process? Is it so terrible to have something off by 0.5 or 1dB?

Post Reply