Hi,
LM4550 codec: there is always "white" noise at the output, no matter > what. Adam will try changing decoupling capacitor models to see if it > makes an improvement. Right now, there is no known solution to reduce > the amount of noise present with the LM4550. > In order to lower ripples on supplies supposedly from 4V3 and 3V3 loops: I replaced for: a) low ESR 1000 mOhms AVS TPSR106K006R1000 on C235 and C34 10uF. b) low ESR 18 mOhms Kemet A700V476M006ATE018 on C133 47uF. (since AC97 signals supplied from bank0 VCCO 3V3) The white noise is still without obviously leveling down, so we'll keep rc2 without changing capacitors on rc3. > WM9707 codec: after replacement with this new chip, weird behaviour and > noises were observed. I have tracked these problems down to the content > of the padding AC97 slots: the WM9707 requires them zeroed out, but the > LM4550 does not. The AC97 controller in the FPGA used to fill those > slots with whatever was present on the CPU bus at the time of emission - > which resulted in the noises being dependent on software activity. This > is now fixed in: > > https://github.com/lekernel/milkymist/commit/1ebbb92f837c30534ecc003ace33d0302861dc20 > After this fix is applied, the audio output with the WM9707 codec is > noise free, so we at least have a fix for the audio noise for the RC3 > boards. > > Using the otherwise footprint-compatible WM9707 would need a few PCB > changes: > * DNP 1M resistor on crystal > * add capacitor on pin 33 > * add capacitors on pin 32 > * route SPDIF to the internal connector (optional) > * DNP the parts for the nonexistent headphones amp > See the datasheet at: > > https://github.com/lekernel/milkymist-datasheets/blob/master/Audio/WM9707.pdf?raw=true > http://en.qi-hardware.com/wiki/File:M1_rc2_0x1e_LINE_OUT_R_scopes_no_patches.ogv To see this ogv from LM4550B, the period (17 ~ 25s) shows white noise is 500mV. After used WM9707; confirmed it's lower than 100mV, so we will apply h/w patches above. -adam
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