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Updated:
05/18/04.
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Who can explain the differences in headroom between "normal pro" use of digital tape and Digibeta? (and other Digital Audio Storage Media, like DAT, MD).
In the old days of Analog you could read your levels with a normal TRUE RMS Voltage Meter.
0dBμ was 0,775 Volt between Hot and Ground. over 600 Ohms.(It dissipated 1 milliwatt of power, hence the Greek mu, for milli).
In active driven balanced mode this results in a voltage of 1,55 V between "+ / Hot / Positive" and "- / Cold / Negative" wires of a balanced or "symetrical" sound system.
It became the "0dBm" level of "Analogue German PPM", "DIN", whatever you call it, but also the broadcast norm for 100% modulation of Audio in Dutch Television.
Over time the 600 Ohm disappeared from the formula, but the voltage reference stayed.
So is +4 dBm (1,23 V) the 100 % input recording level for professional analogue recorders.
In case of normal audio program material, this was all measured with an integration time; 300 msec. for a VU meter, and 0,4-10 msec. (depending on level changes: greater steps = faster) for a PPM.
In case of steady tones all meters read the same.
(For the following examples we refer to steady tone, 1 kHz signals).
Then came Digital.
Because digital converters are very unforgiving to overload, a lot of headroom is essential.
The norm set by SONY multitracks was that digital clip level (NodigiBitsForSale or 0 dBFS (Full Scale)) is 20 dB above the analogue working level of +4dBm.
This level / meter correlation is the same trough my Yamaha O2R (when I put out a tone of 1,23 V to the analogue outputs, the (digital) meters read -20 dB).
But when you put a signal of +6 dBm (1,55 V) into a DigiBeta Video Tape Recorder you see a headroom of 9 dB. (-9 dBFS) (Remember that it should be 18!).
WHO STOLE OUR HEADROOM ???
About other digital levels:
A few weeks ago, I installed a professional DAT machine in a TV broadcast environment and found out that an output level of +6 dBm (1,55V) needed an input of -12 dBFS on an (digital) input meter.
On an pro MD recorder/player I found a reading of -20 dBFS (could be -18 due to display inaccuracy) for an (analogue) output of 1,55V
So there are different norms for digital headroom above my working level of 1,55 volts analogue (so far three; 18dB for multitrack/MD - 9dB for DigiBeta - 12dB for DAT).
Has every tape format it's own headroom?
How about DV - DVCAM - SX - DVCPRO - ??????
What read your machine meters in these formats when you feed them 1,55 volts between pin 2 and 3?
What means "calibrated level" for these machines?
As long as we got to work and interface with analogue equipment like mixers and amplifiers I think manufacturers should clearly publish the digital tape headroom for a given (analogue) in/output level.
What's your opinion about this?
Dick Korporaal
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