[Libav-user] audio filter chain clicks

[Ian Holbrough]

Found it, of course the problem is memory management.

It would seem that avfilter_copy_buf_props reuses the buffer, allocating 
it to the target frame rather than copying it. Unfortunately if you then 
call avfilter_unref_buffer before encoding frame the front of the buffer 
gets trampled on, introducing the clicks.


Ian

On 10/23/12 12:18, Ian Holbrough wrote:
> Hi,
>
> I'm in the process of migrating to libavfilter based resampling using 
> ffmpeg/libav 1.0 and the audio filter chain seems to be introducing 
> audio clicks.  Setting up a passthrough chain with...
>
>     AVFilterContext* last = NULL;
>     filterAudGraph = avfilter_graph_alloc();
>
>     char args[255];
>     snprintf(args, 255, 
> "time_base=%d/%d:sample_rate=%d:sample_fmt=%s:channel_layout=0x%llx",
>         1, inpAudioCodec->sample_rate, inpAudioCodec->sample_rate,
>         av_get_sample_fmt_name(inpAudioCodec->sample_fmt), 
> inpAudioCodec->channel_layout);
>
>     filterAudInp = tsUtilsInitFilter("abuffer", "audInput", args, 
> NULL, filterAudGraph);
>     last = filterAudInp;
>
>     const enum AVSampleFormat formats[] = { AV_SAMPLE_FMT_S16, -1 };
>     AVABufferSinkParams* params = av_abuffersink_params_alloc();
>     params->sample_fmts = formats;
>
>     filterAudOut = tsUtilsInitFilter("ffabuffersink", "audOutput", 
> NULL, params, filterAudGraph);
>     av_free(params);
>
>     avfilter_link(last, 0, filterAudOut, 0);
>
>     avfilter_graph_config(filterAudGraph, NULL);
>
> ...where...
>
> AVFilterContext* tsUtilsInitFilter(const char * filterName, const 
> char* name,
>     const char* arguments, void* opaque, AVFilterGraph* graph)  {
>
>     AVFilter* filter = avfilter_get_by_name(filterName);
>
>     AVFilterContext* context = NULL;
>     int ret = avfilter_graph_create_filter(&context, filter, name, 
> arguments, opaque, graph);
>     if (ret < 0) {
>         die("tsUtilsInitFilter - failed to create filter context 
> <%s>", name);
>     }
>
>     return context;
>
> }
>
> ...however, when a frame is added (I believe the input sample format, 
> rate and channel layout is compatible with the required output) who's 
> first few samples look like this...
>
> 00000000 00000000 00000000 00000000 00000000 00000000 00000000 
> 00000000 00000000 00000000 00000000 00000000 00000000 00000000 
> 00000000 00000000
> 00000000 00000000 00000000 00000000 00000000 00000000 00000000 
> 00000000 ffffffff 00000000 00000000 00000000 00000000 00000000 
> 00000000 00000000
> 00000000 00000000 00000000 00000000 00000000 00000000 00000000 
> 00000000 00000001 00000000 00000001 00000000 00000000 00000000 
> 00000000 00000000
> 00000000 00000000 00000000 00000000 00000000 00000000 00000000 
> 00000000 00000000 00000000 00000000 00000000 00000000 00000000 
> ffffffff 00000000
> ffffffff 00000000 00000000 00000000 00000000 00000000 00000000 
> 00000000 00000000 00000000 00000000 00000000 00000000 00000000 
> 00000000 00000000
> 00000000 00000000 00000000 00000000 00000000 00000000 00000000 
> 00000000 00000000 00000000 00000000 00000000 00000000 00000000 
> 00000000 00000000
>
> ...upon retrieving the same frame from the chain the first 4 samples 
> look somewhat different...
>
> *000063f0 ffffb628 00002ff8 000008d9* 00000000 00000000 00000000 
> 00000000 00000000 00000000 00000000 00000000 00000000 00000000 
> 00000000 00000000
> 00000000 00000000 00000000 00000000 00000000 00000000 00000000 
> 00000000 ffffffff 00000000 00000000 00000000 00000000 00000000 
> 00000000 00000000
> 00000000 00000000 00000000 00000000 00000000 00000000 00000000 
> 00000000 00000001 00000000 00000001 00000000 00000000 00000000 
> 00000000 00000000
> 00000000 00000000 00000000 00000000 00000000 00000000 00000000 
> 00000000 00000000 00000000 00000000 00000000 00000000 00000000 
> ffffffff 00000000
> ffffffff 00000000 00000000 00000000 00000000 00000000 00000000 
> 00000000 00000000 00000000 00000000 00000000 00000000 00000000 
> 00000000 00000000
> 00000000 00000000 00000000 00000000 00000000 00000000 00000000 
> 00000000 00000000 00000000 00000000 00000000 00000000 00000000 
> 00000000 00000000
>
> ...the remainder of the frame is identical.  These sample dumps are 
> produced by...
>
>     short* a = frame->data[0];
>     for (int i = 0; i != frame->nb_samples; i++) {
>         printf("%08x ", *(a++));
>         if ((i % 16) == 15) printf("\n");
>     }
>
> ....removing the filter chain and passing the decoded frames straight 
> through to the encoder does not produce clicking.  Any suggestions as 
> to the cause would be gratefully received.
>
>
> Regards
>
> Ian


-- 

Ian Holbrough, Software Engineer
Yospace - Mobile Video Made Easy

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