Re: [FFmpeg-devel] [PATCH v2 8/8] aacdec: add a decoder for AAC USAC (xHE-AAC)

[Lynne via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

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On 19/05/2024 21:39, Marton Balint wrote:
> 
> 
> On Sun, 19 May 2024, Lynne via ffmpeg-devel wrote:
> 
>> This commit adds a decoder for the frequency-domain part of USAC.
>>
> [...]
> 
>>
>> +/* Finish later */
>> +static const enum AVChannel usac_ch_pos_to_av[64] = {
>> +    [0] = AV_CHAN_FRONT_LEFT,
>> +    [1] = AV_CHAN_FRONT_RIGHT,
>> +    [2] = AV_CHAN_FRONT_CENTER,
>> +    [3] = AV_CHAN_LOW_FREQUENCY,
>> +    [4] = AV_CHAN_BACK_LEFT, // unsure
>> +    [5] = AV_CHAN_BACK_RIGHT, // unsure
>> +    [6] = AV_CHAN_FRONT_LEFT_OF_CENTER,
>> +    [7] = AV_CHAN_FRONT_RIGHT_OF_CENTER,
>> +    [8] = 0, /* rear surround left is missing */
>> +    [9] = 0, /* rear surround right is missing */
>> +    [10] = AV_CHAN_BACK_CENTER,
>> +    [11] = AV_CHAN_SURROUND_DIRECT_LEFT,
>> +    [12] = AV_CHAN_SURROUND_DIRECT_RIGHT,
>> +    [13] = AV_CHAN_SIDE_LEFT, // fairly sure
>> +    [14] = AV_CHAN_SIDE_RIGHT, // fairly sure
>> +    [15] = AV_CHAN_WIDE_LEFT, // somewhat confident
>> +    [16] = AV_CHAN_WIDE_RIGHT, // somewhat confident
>> +    [17] = AV_CHAN_TOP_FRONT_LEFT,
>> +    [18] = AV_CHAN_TOP_FRONT_RIGHT,
>> +    [19] = AV_CHAN_TOP_FRONT_CENTER,
>> +    [20] = AV_CHAN_TOP_BACK_LEFT,
>> +    [21] = AV_CHAN_TOP_BACK_RIGHT,
>> +    [22] = AV_CHAN_TOP_BACK_CENTER,
>> +    [23] = AV_CHAN_TOP_SIDE_LEFT,
>> +    [24] = AV_CHAN_TOP_SIDE_RIGHT,
>> +    [25] = AV_CHAN_TOP_CENTER,
>> +    [26] = AV_CHAN_LOW_FREQUENCY, // actually LFE2
>> +    [27] = AV_CHAN_BOTTOM_FRONT_LEFT,
>> +    [28] = AV_CHAN_BOTTOM_FRONT_RIGHT,
>> +    [29] = AV_CHAN_BOTTOM_FRONT_CENTER,
>> +    [30] = 0, /* top left surround is missing */
>> +    [31] = 0, /* top right surround is missing */
>> +};
> 
> Some comment would be nice about the source of this table (which 
> document, which table).
> 
> It looks very similar to the ISO channel positons used in mov_chan. I 
> think we follow this mapping in most cases:
> 
> Left  Surround is SIDE_LEFT
> Right Surround is SIDE_RIGHT
> Rear Surround Left  is BACK_LEFT
> Rear Surround Right is BACK_RIGHT
> 
> So in your table [4] and [5] should be SIDE, [8] and [9] should be BACK. 
> [26] can be AV_CHAN_LOW_FREQUENCY_2, we do have that.
> 
> Yes, Left/Right Surround and Left/Right Side Surround will be the same, 
> but those are not present in commonly used layouts at the same time.
> 
> Regards,
> Marton
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Source of the table is ISO/IEC 23003-3, Table 74 — bsOutputChannelPos:

0 L left front FL front left
1 R right front FR front right
2 C center front FC front centre
3 LFE low frequency enhancement LFE1 low frequency effects-1
4 Ls left surround LS left surround
5 Rs right surround RS right surround
6 Lc left front center FLc front left centre
7 Rc right front center FRc front right centre
8 Lsr rear surround left BL back left
9 Rsr rear surround right BR back right
10 Cs rear center BC back centre
11 Lsd left surround direct LSd left surround direct
12 Rsd right surround direct RSd right surround direct
13 Lss left side surround SL side left
14 Rss right side surround SR side right
15 Lw left wide front FLw front left wide
16 Rw right wide front FRw front right wide
17 Lv left front vertical height TpFL top front left
18 Rv right front vertical height TpFR top front right
19 Cv center front vertical height TpFC top front centre
20 Lvr left surround vertical height rear TpBL top back left
21 Rvr right surround vertical height rear TpBR top back right
22 Cvr center vertical height rear TpBC top back centre
23 Lvss left vertical height side surround TpSiL top side left
24 Rvss right vertical height side surround TpSiR top side right
25 Ts top center surround TpC top centre
26 LFE2 low frequency enhancement 2 LFE2 low frequency effects-2
27 Lb left front vertical bottom BtFL bottom front left
28 Rb right front vertical bottom BtFR bottom front right
29 Cb center front vertical bottom BtFC bottom front centre
30 Lvs left vertical height surround TpLS top left surround
31 Rvs right vertical height surround TpRS top right surround

Third field is "Loudspeaker position", last field is "Loudspeaker
position according to IEC 100/1706/CDV/IEC 62574 (TC100)", each prefixed 
with an abbreviation.

I've added the source to the table comment in the code.

I've also fixed the SIDE/BACK/LFE2 issue in my github repo I linked earlier.

Thanks.

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