num = st->codec->block_align;
den = st->codec->sample_rate * st->codec->channels;
av_reduce(&den, &num, den, num, (1UL<<31)-1);
num = st->codec->block_align;
den = st->codec->sample_rate * st->codec->channels;
av_reduce(&den, &num, den, num, (1UL<<31)-1);