[quote user="Vojtech Bubnik"]
Hello.
I am experimenting with the TLV320AIC3254 evaluation kit. My target goal is to produce a low power shortwave radio, where most of the processing is done by the 3254. Currently I am having trouble to understand how the ADC processing works.
I am confused about which of the processing blocks are executed on the miniDSP and which are standalone hardware modules. I think the SLAS549A should be improved to put stress on which modules are mutually exclusive with the miniDSP. For a long time I really thought many of the blocks are hardwired next to the miniDSP and I was overjoyed about it. It took me a while to find the following line in the registry table:
Page 0 / Register 61: ADC Signal Processing Block Control Register:
0 0000: The ADC miniDSP will be used for signal processing
Now I believe I got it and understand that many of the processing blocks including the decimation filters of the ADC path are done purely on the miniDSP, right?
Section 5.9 - "ADC Decimation Filtering and Signal Processing" describes processing blocks with the decimation filters A/B/C and the frequency characteristics of the A/B/C filters are given in detail. When adding an AD input block into the PPS worksheet, what decimation filter will be used?
Where are the CIC decimation filters positioned in the whole picture? I suppose they are hardware CIC filters outside the miniDSP, but the A/B/C filters are implemented using the miniDSP?
When I set the ADC preamp to +20dB, I found out that no input signal is mapped to around 1/2 of the ADC span. That means at +20dB I get a heavy DC component and 6dB lower dynamic range. I believe at +30dB preamp the DC offset was out of the ADC range. Is it normal or is there any way to tame the preamp offset?
Does the ADC AGC control the ADC preamp only? Or does it control the ADC digital volume control as well?
What is the digital volume control anyway? It says it spans from -12dB to +20dB. Is it just a multiplication inside the miniDSP? Or is it a hardware multiplier between the CIC filter and miniDSP? How does setting the digital volume control different from 0dB affect the dynamic range and noise?
Thank you for enlightment,
Vojtech
[/quote]
Hi,
Generally speaking from having used the AIC3254, once you begin making use of the miniDSPs (A & D) you will not need to go back to the other processing blocks. The only suprise I got that disapointed me is that you cannot use the other processing blocks at the same time as the miniDSPs so you have to sometimes redesign them in purepath etc. (one thing that comes to mind is the Beep tone). So basically from the register configurations you will either use the miniDSPs, or the other processing blocks, but not all at once ...
The choice of decimation filters with regards to the miniDSP is done in what "Framework" you chose and the associated decimators and interpolators, when you design your purepath code. This is again, is done in purepath according to some of the set rules, depending on the sampling rate you need etc. You may want to check out the info on the frameworks and the decimators/interpolators available.
Not sure why you seem to get a dc component, but bare in mind that depending on the choice of inputs, differential or common mode, you will usually get a DC component on the IN1, IN2 and IN3 pins ... These signals are alsways decoupled by the 0.1uF ceramics from the microphone sources etc. The "pre-amp offset" I believe is related to the common mode settings of you PGA and these values can be adjusted slightly, but I don't belive they can be tuned to zero...
Digital volume is used to control the playing volume. I've always used this tightly coupled to a (-) & (+) key to tune the volume down or up of the audio signal. The code for doing this you will need to work out according to your requirements. I believe that it is the gain setting of some amplifier stage in the codec, but it may be a digital gain as you call it. The HP gain also needs to be adjusted according to your requirements, and then left at the fixed value that gives you the best ouput signal volume vs distortion vs noise ...
Hope this helps in some way.
Cheers, Mike