CODEWITHSUNDEEP
audiotimeffmpegstreamlibav
xtingray
xtingray

Reputation: 503

Libav API: How to modify the start time of an audio stream?

Currently, I am working on a new feature for my software using the Libav API. I was able to merge a video file with and audio file, the output is an MP4 file and the source code works perfectly.

Right now, I have a new requirement: the start time for the video and the audio streams are the same. Both start at 00:00. My next challenge is to add an option to shift the start time for the audio stream based on a variable. For example, if audio start variable is equal to 10 seconds, then the audio stream should be played at time 00:10.

So I wonder, what is the best approach to implement this feature? Should I insert silent packets before the audio start time is reached? Or should I modify the timestamp information for every audio stream packet before it is written into the output container?

Here is the piece of code I use to write the audio stream into the MP4 file. Right now it works like a charm, but I guess this is the place where I should implement the new requirement. I will appreciate any suggestion.

while (1) {
    AVStream *in_stream;

    int ret = av_read_frame(audioInputFormatContext, pkt);
    if (ret < 0)
        break;

    in_stream  = audioInputFormatContext->streams[pkt->stream_index];
    pkt->stream_index = outIndex;

    AVRational out_time_base = audioStreamList.at(i)->time_base;

    // copy packet
    pkt->pts = av_rescale_q_rnd(pkt->pts, in_stream->time_base, audioStreamList.at(i)->time_base,
                                static_cast<AVRounding>(AV_ROUND_NEAR_INF|AV_ROUND_PASS_MINMAX));
    pkt->dts = av_rescale_q_rnd(pkt->dts, in_stream->time_base, audioStreamList.at(i)->time_base,
                                static_cast<AVRounding>(AV_ROUND_NEAR_INF|AV_ROUND_PASS_MINMAX));
    pkt->duration = av_rescale_q(pkt->duration, in_stream->time_base, audioStreamList.at(i)->time_base);
    pkt->pos = -1;

    ret = av_interleaved_write_frame(formatContext, pkt);
    if (ret < 0) {
        fprintf(stderr, "Error muxing packet\n");
        break;
    }
    av_packet_unref(pkt);
}

Upvotes: 0

Views: 291

Answers (1)

xtingray
xtingray

Reputation: 503

The best way to modify the start time of any audio stream using the Libav API is by implementing a "adelay" filter. Here is a little piece of code showing how to use it:

AVFilterContext *adelay_ctx;
const AVFilter  *adelay;
char args[512]; // This variable contains the filter parameters
int error;

adelay = avfilter_get_by_name("adelay");
if (!adelay) {
    av_log(NULL, AV_LOG_ERROR, "Could not find the adelay filter.\n");
    return AVERROR_FILTER_NOT_FOUND;
}

int delay_time = 8000; // delay in milliseconds 
snprintf(args, sizeof(args), "delays=%d:all=1", delay_time);
error = avfilter_graph_create_filter(&adelay_ctx, adelay, "adelay", args,
                                     NULL, filter_graph);
if (error < 0) {
    av_log(NULL, AV_LOG_ERROR, "Cannot create audio adelay filter\n");
    return error;
}

Upvotes: 0

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