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Question about Tape Deck and dB standards.

knl

knl

Active member
Joined
Jul 12, 2023
Messages
51
Location
South Africa
I have a few questions for the tape heads out there.

Do Cassette Decks (or Reel to Reel) amplify the input signal at all? In other words, if you turn the input level to Max, do the recording electronics see the input signal without any additional amplification?

In addition, what standard are used for DB in cassette decks and are there differences (do 0dB refer to dbV, 1V or dBm which is 0.775V).

In other words, if I want to calibrate (in a general case) my decks dB meters, can I just adjust recording level to Max, input a 0.775V signal from an Audio Generator and adjust the meter to 0dB.

Obviously the correct method is per the Service Manual, but I do not have access to the reference tapes specified in most cases.
 
I tested this many years. It does amplify. Wrt to db, it depends on the brand. There is a standard but the variance on the standard by brand has quite a bit of float, +-10db. That can make a huge difference.

So if your machine is in perfect working condition, and you are calibrating, you should follow whatever the service manual says about the setup.

You can use a digital device, like Ipod or MP3 player, on the input of your player to calibrate. Ypu have to know what the voltages etc are and adjust the digital player accordingly. This could take longer than using reference tapes but works.
 
I recently calibrated a dec VU meters as they were differing by a lot. VU is a standard in and of itself. On this specific deck, had to supply a signal at a know Voltage peak to peak, not RMS, and then set the input controls midscale. Then adjust the VU to 0DB display.

Basically you have to follow the setup as per the service manual.

Groetnis
 
pieter78 said:
I tested this many years. It does amplify. Wrt to db, it depends on the brand. There is a standard but the variance on the standard by brand has quite a bit of float, +-10db. That can make a huge difference.

So if your machine is in perfect working condition, and you are calibrating, you should follow whatever the service manual says about the setup.

You can use a digital device, like Ipod or MP3 player, on the input of your player to calibrate. Ypu have to know what the voltages etc are and adjust the digital player accordingly. This could take longer than using reference tapes but works.
Click to expand...
Thank you.. I do have an audio tester, so can apply the exact voltages. I was just not sure about what position the Input level knob should be. Max or centered.
 
I agree with Sarel. The service manual of the tape machine will provide most of the settings to calibrate the specific tape machine.

This is so true:
On this specific deck, had to supply a signal at a know Voltage peak to peak, not RMS, and then set the input controls midscale. Then adjust the VU to 0DB display.
Lots of the audio signal generators do not provide both (RMS/V peak tp peak), Certain generators can select one or the other.

Two very good books:
Questions & Answers about Tape Recording by Herman Burstein
and
Modern Recording Techniques by Robert E Runstein
 
Remember decibels are just a ratio they have no value unless a reference is specified:
0.775V across is 600Ohms is 1mW and that is 0dBm
0.775V across any (assumed high) resistance is 0dBu
1V across across any (assumed high) resistance is 0dBV
Real VU meters usually read 0dB for +4dbu input (approximately 1.3V in) this is considered 'professional' level ('consumer' level is approximately -8dBu or 316mV)
A VU meter is defined by the speed of it's readings; how fast it reacts to an input voltage and how long it takes to decay from that voltage. A peak meter reacts instantaneously and decays at a rate usually set to make it easier to read. VU meters' characteristics were designed to make the meter display an 'average' of the signal because this is more akin to our perception of a signals volume (hence the name Volume Unit meter) as opposed to the blatant accuracy of a peak meter. A VU meter therefore gives an indication of how "loud" a signal is and cannot be used for accurate measurement.
Tape does not distort immediately - it does not "clip" - hence you can briefly "go into the red" for peaks. The implicit understanding of a VU meter was that it would display a signal's level but the peak of that signal would be much higher than that displayed. So, it is always best to calibrate your tape deck meters according to the manual since they could well be peak meters, they may not be genuine VU meters (wrong ballistics) or the tape recommended for the machine had distortion characteristics at a certain level and that would be deemed 0dB on the display.
 
As a matter of interest the BBC called VU meters "virtually useless" 🤭 - they were primarily interested in broadcasting with FM having very defined maximum system limits and live transmissions meaning no chance of correcting any errors - in this case overloads and/or distortion, so peak meters allowed an instant indication of overload. Digital recording has instantaneous clipping at full scale (usually 2V RMS but depends on the ADC) which is 0dBFS - so only very fast peak meters are useful there.
 
Booby is a rockstar so to speak, mixed many o them. Do have some of his books, a genius for sure....

Groetnis
 
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