[FFmpeg-devel] [PATCH 3/3] avformat/dashenc: always attempt to enable prft on ldash mode

[Jeyapal, Karthick]

On 2/19/20 4:21 PM, Thilo Borgmann wrote:
> Am 19.02.20 um 06:18 schrieb Jeyapal, Karthick:
>>
>> On 2/18/20 9:43 PM, James Almer wrote:
>>> Signed-off-by: James Almer <jamrial at gmail.com>
>>> ---
>>>  libavformat/dashenc.c | 5 +++++
>>>  1 file changed, 5 insertions(+)
>>>
>>> diff --git a/libavformat/dashenc.c b/libavformat/dashenc.c
>>> index b910cc22d0..045d2f4df6 100644
>>> --- a/libavformat/dashenc.c
>>> +++ b/libavformat/dashenc.c
>>> @@ -1395,6 +1395,11 @@ static int dash_init(AVFormatContext *s)
>>>          c->frag_type = FRAG_TYPE_EVERY_FRAME;
>>>      }
>>>  
>>> +    if (c->ldash && !c->write_prft) {
>>> +        av_log(s, AV_LOG_INFO, "Enabling Producer Reference Time element for Low Latency mode\n");
>>> +        c->write_prft = 1;
>>> +    }
>>> +
>> PRFT elements has a significant bitrate overhead, especially in streaming mode when each frame is a moof fragment.
>> In terms of percentage of stream's bitrate this overhead will be a significant % for lower bitrate streams(such as audio streams).
>> For any application which does not need PRFT this is an unnecessary wastage of bits. 
>> Hence, I would advise against enabling PRFT without user control.
>
> Latest to-become spec for low latency mode declares it mandatory [1].
I see. Now I understand the motive behind this change. 
>
> I see your point, though. What significance would this actually have, can you provide some numbers / examples?
Sorry. I worked on this bitrate overhead optimizations around a year back. Hence, I don’t have the numbers with and without PRFT handy. 
But I do have the final overhead numbers (without PRFT) for an audio stream. 
CMAF Muxer overhead (for an AAC-LC codec) by Sampling frequency
16000 Hz - 14 Kbps
24000 Hz - 20 Kbps
32000 Hz - 28 Kbps
48000 Hz - 40 Kbps

At 48KHz, the overhead due to CMAF was 40 Kbps which was significant by itself. 
My random guess is that PRFT would add another 10Kbps - 20Kbps. But I could be wrong here, as I don’t remember exactly.

> If that turns out to be actually significant, I don't know if we would prefer an override option to disable it and produce non-conformant manifests or live with the overhead.
Yes. Having an option to control this behavior would be useful.
>
> -Thilo
>
> [1] https://dashif.org/docs/DASH-IF-IOP-CR-Low-Latency-Live-Community-Review-Dec-2019.pdf
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