| Jeff Sayers wrote on Fri, 03 December 2004 15:31 |
Greetings,
I am piecing together a very modest home studio and am considering either the Rosetta 200 or Lavry Blue for my conversion. Popular opinion seems to be that the Lavry is perhaps more natural sounding than the Rosetta, and my only reservation about purchasing the Lavry is its lack of s/pdif for connecting with my Digi 002R. I have seen that Mercenary offers AES - s/pdif transformers, I would just like some confirmation that using such a transformer would not degrade or in any way negatively impact the audio signal. Maybe this is a silly question, but I'm not a digital guru, just someone who wants the best for what I can afford. Thanks.
Jeff Sayers
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In principle, an AES-SPDIF transformer will work just fine. Assuming we are talking about the same device (made by Neutrix), the Mercenary AES/SPDIF transformer should do fine, but you will need to convert the output side from BNC to RCA. Such BNC to RCA adaptors are easy to come by, they cost about $2.5 or so.
The rest of the message will explain why you will have NO DEGRADATION. The concept is:
With analog signals, the shape of the wave itself represents the information content (the audio). A slight alteration of the wave will change the signal (the audio).
With digital, the information is converted to numbers and the signal itself does not need to be precise to carry the numbers. One can “distort” a digital signal by a huge amount, to a point, and “nothing bad will happen”. You pass that point, and the communication link breaks down. You stay in the “safe zone” and all is fine.
Once the AD conversion is done, the data is “cast in stone”. You are now in the digital world, the “safe zone”. Let me explain:
Say you add a 1mV (millivolt) of noise to your analog signal, or 1nSec (nano second) jitter to your conversion clock. Such disturbance would cause trouble. But that all took place in the analog world. Once you converted to digital, the picture is different.
The digital data is “just a bunch of” 0 and 1 “patterns”. One specific DIGITAL sample can look like 111000010010010100100101 another sample may be for example 1100000100100100101001001
The task of storing or transferring such samples is highly immune to disturbance. Do you really care how the patterns get registered on your hard drive? All you care about is keeping each 0 as a 0 and each 1 as a 1.
Now, here is the advantage of digital. The same 1mV noise or 1nsec jitter will do NO HARM to the digital signal. A typical digital logic recognizes a 0 as about -.5V-2V range and a 1 as 3V-5.5V. So if I send you a 4V signal with 1V noise (1000mV) you will never know! If my 0 is bouncing by 1V you will not know either. A 0 is still a 0 and a 1 is still a 1.
In terms of interface clock jitter, lets say we use 96KHz sample rate, so the time between samples is about 10uSec. You do need to “squeeze” 2 channels of data (24 bits each) and some other bits… a total of 64 bits per sample. Each 0 or 1 lasts for about 160nsec. So you “aim for the center” and you have a +/-80nsec allowable timing error range. This is a only a 6MHz serial transmission rate.
AS YOU MUST ALREADY KNOW BUT I WILL SAY JUST TO BE THOROUGH, the advantage of digital is it’s immunity to disturbances and interference, and the ability to be stored in memory. One cannot do it with analog. Analog is great, but the vinyl scratches and wears out, the tape demagnetizes over time, a copy is always worse than the original, you are susceptible to the slightest component variations and signal disturbances.
Digital is what makes CD’s great. Digital enables you to send emails over phone lines,
And the outcome looks the same for slow transmission as it does for fast transmission. Some noise on the line will not add fuzz to the letters. Digital is about yes or no. It “takes out the requirement for precision”.
So your issue with digital transfer is reduced to answering the question: does it work? Is each 0 seen as a 0? Is each 1 seen as a 1?
At 6MHz serial rate for 96Khz, the answer is yes. It works. In fact, you can get make it work in most cases with an XLR to RCA adaptor and no transformer, if the cable length is only a few feet. True, the XLR is sending 3-5V and is expected to see a 110 Ohm load. The RCA is a designed for a 75 Ohm cable, and .4V signal. But it is DIGITAL, so it is very “forgiving”. The digital audio transmitter and receiver IC’s are mostly the same devices used for handling both AES and SPDIF signals. The impedance is a non issue for short cable length. I am not against a transformer, it is a “closer to the intended design goal” and allows longer cables. Chances are that at 3-4 feet of reasonably good cable, a transformer will not make a difference.
Once the AD conversion is done, the data is “cast in stone”. You are now in the digital world, the “safe zone”…
Regards
Dan Lavry
