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
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.
Thanks again for your time and for the detailed explanation.
As I understand it now, we are providing headroom to accommodate the dynamic content but in the process utilizing less of the ADC full range for normal content there by losing out on resolution. I hope I am correct.
So it seems to me that adjusting the ADC input resistors of the ADAU1701 processor will always be a compromise between resolution of normal signals and having headroom to accommodate the dynamic signals. Right?
I believe that the safest way, at this point is to have the values for the ADC_RES and the ADC input resistors as done on the eval board which is 18k and 8K respectively.
Do let me know if you have any other thoughts.
Yes you are correct. It is a tradeoff. This is why it is somewhat subjective and it also depends on your application. This is why I expect that others may have an opinion on this that differs from mine.