Channel mapping for 3/1 audio program

[Rohit-Khali]

Hi,
For a 3/1 audio program having 4 channels as L, C, R, S, what should be the correct channel mapping for channel "S"?
As per ebur128.h 'enum channel' Ls and Rs are listed. What is the corresponding mapping for "S" in terms of Ls and Rs.

Thanks,
Rohit Khali

[jiixyj]

You could try with EBUR128_Mp180 which corresponds to M+180 from here: https://www.itu.int/dms_pubrec/itu-r/rec/bs/R-REC-BS.2051-2-201807-I!!PDF-E.pdf

But in the end, all surround channels are treated pretty much the same, so using EBUR128_Mp180 should yield the same results as using EBUR128_LEFT_SURROUND.

[Rohit-Khali]

Thanks a lot for the response !
[But in the end, all surround channels are treated pretty much the same, so using EBUR128_Mp180 should yield the same results as using EBUR128_LEFT_SURROUND.] I do not see EBUR128_Mp180 in the list of channels for which "channel_sum *= 1.41" is being done in ebur128_calc_gating_block() however EBUR128_LEFT_SURROUND (EBUR128_Mp110) and EBUR128_RIGHT_SURROUND (EBUR128_Mm110) are there. So want to cross check before I use EBUR128_Mp180.

[jiixyj]

Ah, you're right, M+180 is not treated the same as the left/right surround channels.

If we look at Annex 3 of https://www.itu.int/dms_pubrec/itu-r/rec/bs/R-REC-BS.1770-4-201510-I!!PDF-E.pdf, only channels with an absolute elevation angle < 30 degrees and absolute azimuth between 60 and 120 degrees get the weight "1.41". All other channels keep a weight of "1".

So in your case, that would mean that the M+180 channel has a weight of "1".

[jiixyj]

So for completeness, the only channels that get a weight of 1.41 are exactly:

• M+060
• M-060
• M+090
• M-090
• M+110 (also called EBUR128_LEFT_SURROUND)
• M-110 (also called EBUR128_RIGHT_SURROUND)

[Rohit-Khali]

Perfect. Thanks a lot!
Noticed a typo in https://github.com/jiixyj/libebur128/blob/master/ebur128/ebur128.h. Line no: 51
EBUR128_Up045, /< itu U+045 */
_EBUR128_Um045, /< itu U-030 */_ --> itu U-045

I have few other minor queries raised as #112, #113, #114 and #116. Whenever you get time, please try to have a look.
Thanks in advance.