17-11-2018, 11:11 PM
Hi Stephen and Trevor.
Thank you both.
Hi Jeffrey
How to implement audio modulator had me stumped for a while. As you have said getting the correct carrier level was the problem. An amplifier is required to amplify the incoming audio to close to 3.3 V to get the full range out of the ADC. I have used an TS971 op amp for this purpose. It's output is DC coupled to the input of the ADC. The Quiescent voltage at the opamp's output will set the audio carrier level. So by biasing it so that it is at half rail voltage should set the audio carrier to be 6 dB down on peak white. Well that's the theory anyways but it has to be tested yet!
The video levels at present are not ideal for the modulator. They are (peak white) 216 + (sync) 92 = (total) 308. Scaling the values to have a total of 512 would be much better and might improve the the signal to noise. The scaling probably can be done within the oversampler but I will have to think about that a little more.
Frank
Thank you both.
Hi Jeffrey
How to implement audio modulator had me stumped for a while. As you have said getting the correct carrier level was the problem. An amplifier is required to amplify the incoming audio to close to 3.3 V to get the full range out of the ADC. I have used an TS971 op amp for this purpose. It's output is DC coupled to the input of the ADC. The Quiescent voltage at the opamp's output will set the audio carrier level. So by biasing it so that it is at half rail voltage should set the audio carrier to be 6 dB down on peak white. Well that's the theory anyways but it has to be tested yet!
The video levels at present are not ideal for the modulator. They are (peak white) 216 + (sync) 92 = (total) 308. Scaling the values to have a total of 512 would be much better and might improve the the signal to noise. The scaling probably can be done within the oversampler but I will have to think about that a little more.
Frank
