[Libav-user] Audio quality loss while encoding

[Paul B Mahol]

On 4/25/13, Brad O'Hearne <brado at bighillsoftware.com> wrote:
> On Apr 24, 2013, at 4:54 PM, Paul B Mahol <onemda at gmail.com> wrote:
>
>> You call av_samples_alloc_array_and_samples() with align set to 0, which
>> means
>> channel data planes are aligned along 32 byte boundary. Is this really the
>> case?
>>
>> Set align argument to 1 if to make them aligned along 1 byte boundary.
>
> Here is the information on the sample buffer received from QTKit which is
> being used to fill the source data array:
>
> 2013-04-24 17:06:58.653 QTFFmpeg[2732:d407] Linear PCM, 32 bit little-endian
> floating point, 2 channels, 44100 Hz
> 2013-04-24 17:06:58.654 QTFFmpeg[2732:d407] Bytes per frame:  4
> 2013-04-24 17:06:58.654 QTFFmpeg[2732:d407] Frames per packet:  1
> 2013-04-24 17:06:58.655 QTFFmpeg[2732:d407] Bits per channel: 32
> 2013-04-24 17:06:58.655 QTFFmpeg[2732:d407] Is packed? YES
> 2013-04-24 17:06:58.655 QTFFmpeg[2732:d407] Is high aligned? NO
> 2013-04-24 17:06:58.656 QTFFmpeg[2732:d407] Channels per frame: 2
>
> So to answer your question, provided that I've understood it correctly,
> samples are packed, therefore should be aligned on the full 32-bit boundary,

Its byte and not bit.

Another thing, you overwrite sourceData[x] pointers, causing memory leak.

> and thus align should be set to 0. But for the sake of curiosity, I changed
> align to 1 (for all functions with align) and it made no difference in the
> audio distortion...same as before. I then tried setting the align parameter
> to 1 on only the filling of the source data array -- distorted audio still,
> but seemed to cut the whole track short.
>
> To the best of my knowledge, a packed sample buffer should not need high
> alignment (which I presume is what you mean by the 1 byte boundary). If I've
> misunderstood your question, let me know.
>
> Thanks,
>
> Brad
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