Need suggestions for correct full-scale ADC values


This question is about giving the correct value to the ADC input resistor of ADAU1701. However, since it is more of a general audio application question, I thought of posting it here to get suggestions from a wider audience.

I will be using the ADAU1701 SigmaDSP processor in home application as well as in Pro applications.

Will it be correct to assume an input full scale value of 0.316V (-10dbV) for home/ domestic use (with Media players, STB audio output etc.) and 1.228V (+4dbU) for PRO use (Mike/ AUX/ DJ mixer)?

Do suggest you you have any other views/ opinions.

Thanks in advance!!

Note: Since the processor already has a 2k resistor internally, I'll have to re-adjust the input full scale for Home

SigmaDSP specific note added at the end
[edited by: KKSL at 5:07 PM (GMT 0) on 11 Feb 2019]
  • Hello KKSL,

    First I want to say to anyone else reading this, please chime in and give your opinion. His question is a little subjective so the more answers the merrier.

    So remember that full scale in the ADC is full scale digital which is also written as 0dBFS. Above this is only distortion. The consumer and pro levels you referenced are nominal levels not maximum. There is always lots of headroom above this nominal level before clipping. This discussion has been around since the early use of digital audio back in the late 70's and early 80's. (I am old enough to remember all this. ) There was a lively discussion as to what level +4dBu should be in digital. This answer always depended on the type of material a person was recording. So the standards I recall were where +4dBu analog in would give you between -14 and -20 dBFS. Most people used -16dBFS with some rock and pop albums using +14 and classical at -18 or -20dBFS. Then you had the crowd that always wanted to use as many of the 16 bits in a CD as possible. So they cranked up the mix until the peaks of the song were right at 0dBFS. This made for wildly different levels between one CD and another. This still persists today!

    So to answer your question, Since I see professional interfaces clipping around +21 to +24 dBu, I would set up the ADC clipping to be at +24dBu. That is 10 dB of headroom. Since most mastered music is not that dynamic that should be enough. The early discussions I referenced were for recording studios where the audio was not all compressed like it is in the final product. So they needed the 14 or 16 dB of headroom.

    Dave T

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