Any news on using the STM32F4 with codec2? I just got my hands on a board
and would love to try it with something awesome like codec2. I believe it
would make a great short-range HT with one of those small 433MHz data
radios like the RFM22 from Hope RF.
--------------8<--------------8<-------------
*Ronan Paixão*
2011/11/7 Peter Lawrence <[email protected]>
> FYI, I grafted the ST Micro PDM library that Ronan Paixão found into
> my Codec2 Cortex-M4 test code.
>
> The library seems to consume about 28uS or 33uS of CPU for every 1mS
> of audio input, for 64x or 80x decimation respectively. So, it adds
> about 0.5 to 0.7 mS per 20mS audio frame, or about 10% extra above the
> 256-element length FFT Codec2 routine.
>
> The clarity is better than my previous makeshift PDM to LPCM
> conversion routine, but the microphone still has lots of wideband
> noise. This really dilutes the Codec2 results. Some sort of DNR
> might help.
>
> Just for posterity (or for someone else attempting to use ST's PDM
> library):
>
> The ST PDM library operates on 1 mS worth of audio, and decimates the
> PDM stream by either by 64 or 80 times.
>
> What this means is that the minimum possible LPCM rate out is 16kHz
> (twice the Codec2 rate). This is forced by the MP45DT02 datasheet,
> which indicates a minimum clock rate of 1 MHz is required.
>
> However, since the library has a configurable low-pass frequency, one
> can then down-sample to 8kHz before routing the audio to the codec.
>
> There is a MicGain (or Volume... the ST documentation waffles)
> parameter. 0 appears to be mute. The higher the value, the louder
> the audio, but the louder the noise too.
>
> Although the parameter can go up to 64 in principle, I found anything
> more than 3-4 just made the codec results very distorted. At 1-4, one
> has to speak very loudly directly into the microphone; the results
> seem intelligible, but still nowhere near as clear as the HTS samples.
>
> Also, note that although the AN3998 documentation indicates on page 7
> that PDMFilter_InitStruct only has an Out_MicChannels member, it
> actually has both an In_MicChannels and Out_MicChannels member. Both
> need to be set to valid values for the library to work.
>
> 2011/11/7 Ronan Paixão <[email protected]>:
> > For the record, there is an Application Note about PDM in ST's STM32F4
> page:
> > AN3998: PDM audio software decoding on STM32 microcontrollers (pdf)
> > This is exactly about the MEMs mic on the STM32F4 Discovery board and
> how to
> > convert it to 16-bit PCM. From the introduction:
>
>
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