96 kHz / 88.2 kHz on the iPod Video

[rkjnsn]

I noticed that there was a patch committed back in April that allowed multiple output frequencies to be used on the iPod video, but it only supported up to 48 kHz, as the author wasn't sure if the WM8758 supported anything higher.

I have to questions:
1. According to FS #10086.  "This does absolutely nothing for standard audio playback. There doesn't seem to be any sample rate changing support there, and so software resamples to 44100 if necessary."

Is this still the case?

2. I have seen conflicting information on the WM8758, with some saying that it can indeed go up to 96 kHz. (The DACs in earlier iPods seem to)  Does anyone have any information on this?  Would it be dangerous to test it?

I would like to see this working, and am willing to put some work into making it happen.  I would say that I am a decent C programmer, but I know next to nothing about the hardware side of things (but I want to learn).  I was wondering if anyone would be willing to assist me with this (or tell me it's not possible).

Thank you for your time.

-- Erik

[saratoga]

1)  Yes, the playback engine still resamples everything.

2)  Generally trying to clock hardware differently doesn't damage it, though it may crash rockbox if the settings aren't allowed.

I don't think theres any datasheet for the WM8758, so the obvious thing to do is look at the code, see how its clocked, and see if doubling from 44.1KHz results in sounds playing with the pitch off by 2x. 

Edit:  Misread question 1.

[linuxstb]

I don't think theres any datasheet for the WM8758, so the obvious thing to do is look at the code, see how its clocked, and see if doubling from 44.1KHz results in sounds playing with the pitch off by 2x. 

The driver for the WM8758 was developed using the datasheet for the WM8983 - it's proven close enough for most things (although it claims a hardware EQ, which has been proven to not be present in the WM8758).  So that datasheet should help you set the clocks for different samplerates:

http://www.wolfsonmicro.com/uploads/documents/en/WM8983.pdf

Support for different playback samplerates is something I've long wanted in Rockbox, so it would be great if you worked on it.   

[dreamlayers]

When calculating how to set sample rates, I did calculations for frequencies up to 96 kHz.  They are in a Google Docs spreadsheet.  Note that just two PLL configurations are used and the rest is done via MCLKDIV.  You only need to add to freq_setups and SAMPR_CAPS. 

The only thing that's missing is ADDCTRL_SR_ values for higher frequencies.  That only affects output filtering, which might affect quality but shouldn't prevent setting of higher sample rates. 

I did not try sample rates beyond 48 kHz.  Whoever tries them is doing it at their own risk.