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

[Thilo Borgmann]

Am 20.02.20 um 04:24 schrieb James Almer:
> On 2/19/2020 11:24 PM, Jeyapal, Karthick wrote:
>>
>> On 2/20/20 7:19 AM, James Almer wrote:
>>> On 2/19/2020 9:33 PM, Jeyapal, Karthick wrote:
>>>>
>>>> On 2/19/20 7:05 PM, James Almer wrote:
>>>>> On 2/19/2020 8:50 AM, Jeyapal, Karthick wrote:
>>>>>>
>>>>>> 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.
>>>>>
>>>>> prft is a 32 byte box per dash segment, and segment duration can be
>>>>> configured. A 1 second long segment for a 96kbps 44kHz audio stream with
>>>>> a single moof/mdat pair inside is about 12kb. 32 bytes aren't going to
>>>>> affect it.
>>>> Thanks for your clarification. If the prft is created only once per segment, then it is not a big overhead.
>>>> I had encountered a case where pfrt was getting created once per fragment, and hence was worried.
>>>
>>> Actually, you're right, it's one per fragment. My mistake. But you can
>>> control both fragment count per segment and frame count per fragment in
>>> the dash muxer now, and for low latency dash one fragment per segment of
>>> about 1 second each is recommended for audio streams.
>> If that is the case, I would like to point out that not everybody might choose 1 second per fragment. 
>> Anyone interested in reduced latency and stability would go for 1 frame per fragment. 
>> In our tests(in production environments), 1 frame per fragment provides much smoother and stable behavior at low latency streaming than higher fragment sizes.
>> And in such a case PRFT will add 12Kbps overhead for a 48Khz AAC-LC stream. Hence, I suggest we keep this behavior configurable.
> 
> One audio frame per fragment is a considerable overhead by itself just
> by the excess of moof atoms.

> But in any case, i guess i can skip this
> patch for the time being, while i look for a good way to for example
> configure prft in a per adaptation set basis, or just enable it for
> video streams.

I don't think so. We should not fail the spec on default because of one possible use case.

Since we seem to have found that this can be a significant overhead, we should add an override option to violate the spec (and skip prft) - but in that case also print a warning.

-Thilo


> 
>>>
>>>>>
>>>>> The real overhead is in the CMAF fragmentation/segmentation. Each moof
>>>>> box can be in the hundreds of bytes depending on frame count. The more
>>>>> moof/mdat pairs (AKA CMAF Chunks) are used, the bigger the overhead.
>>>>> Before my recent changes the dash muxer would always make one per frame
>>>>> in streaming mode, which was excessive, especially for audio. But now
>>>>> you can customize it in various ways.
>>>>>
>>>>>>
>>>>>>> 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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