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       HLS-34: Audio Bitrate Scaling

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Issue Type: New Feature 
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Issue Information
====================

Priority: Major					Status:      Open
Resolution:  Unresolved
Project: HLS Stream Creator (HLS)


Reported By: btasker					
Assigned To: btasker

Components: 
	- Transcoding 
	- Adaptive Streams 


Targeted for fix in version: 
	- VNEXT

Labels: Audio, HLS, 

Time Estimate: 0 minutes
Time Logged:   0 minutes


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Issue Description
==================

It's always been a feature of the HLS spec, but isn't something I've really had a use
for.

A user has asked in Github #33 for a switch to set/scale audio bitrates.

This could be implemented in one of 2 ways:

- You can specify an audio bitrate alongside a video bitrate. The audio track in segments
with that video bitrate would have their audio at the specified rate (or native if not
specified)
- A new flag (for sake of argument -a) is added and works a lot like -b. You pass in the
audio bitrates you want (e.g. -a 96,128,384). Audio track is stripped from segments and
broken out into separate audio streams and declared in the manifest

The former is _much_ easier to implement, and gives the operator more control. 

The second is harder to implement, but gives the player more choice. 

Having separate audio tracks also lays the groundwork for possible future enhancements -
like (for example) specifying alternate audio tracks (e.g. German audio instead of English
etc).

It'd also mean we could (potentially) implement the ability to ignore video and only
output audio streams (allowing the script to be used for things like adaptive radio
streams).

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Issue Relations
================
	
	- Github #33 (https://github.com/bentasker/HLS-Stream-Creator/issues/33)



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Activity
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2019-07-26 09:22:09              btasker
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Although responsibility will probably (initially) be pushed onto the encoding operator,
the following should be noted (from
https://developer.apple.com/documentation/http_live_streaming/hls_authoring_specification_for_apple_devices)


-- BEGIN QUOTE --

2.5. Supported multichannel audio formats are:

- Dolby Digital (AC-3)
- Dolby Digital Plus (E-AC-3)
- Dolby Digital Plus with Dolby Atmos

2.6. If Dolby Digital Plus is provided and your streams are delivered to devices that do
not support Dolby Digital Plus then Dolby Digital MUST be provided also. See Audio
Compatibility.

2.7. Stereo audio MAY be provided at multiple bit rates

2.8. Multichannel audio MUST be provided at a single bit rate. (For example, don’t provide
Dolby Digital 5.1 at two bit rates, but you may provide Dolby Digital 5.1 and Dolby
Digital Plus 5.1.)

2.17. A single media segment MUST NOT contain multiple audio streams.

-- END QUOTE --
	
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2019-07-26 10:05:49              btasker
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If we do go the route (even initially) of specifying the audio bitrate within -b my one
requirement for accepting pull requests is laid out on Github:

-- BEGIN QUOTE --

What I would say though, is it would be prudent to have a different delimiter to denote
audio, as it reduces the need to worry about what order things have been specified in
(reducing the scope for operator error).

So something like [VideoBW]-[width]x[height]:[audiobw] rather than splitting with a
hyphen. It doesn't have to be a colon specifically.

-- END QUOTE --
	
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2019-11-23 12:09:13              
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btasker added 'VNEXT' to Fix Version
	
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2019-11-23 12:09:13              
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btasker removed '1.0' from Fix Version