[Libav-user] audio filter chain clicks

[Ian Holbrough]

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
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