No RBSP stop bit is present after the SODB.

[dkozinski]

A NAL unit consists of a NAL header and RBSP.
RBSP contains the SODB, and RBSP trailing bits (RBSP stop bit and zero or more subsequent bits equal to 0) present after the SODB.

According to the ISO/IEC 23094-1:2020 documentation:
"RBSP (raw byte sequence payload) is a syntax structure containing an integer number of bytes that is encapsulated in a NAL unit and that is either empty or has the form of a string of data bits (SODB) containing syntax elements followed by an RBSP stop bit and zero or more subsequent bits equal to 0"

It appears that the RBSP stop bit is not written to the stream after the SODB sequence.

Below, I provided example data extracted from an EVC stream.

NAL unit of type SPS
Length: 21 bytes

00 00 00 15
32 00 80 6B 80 00 00 00 00 00 00 20 05 02 01 E1 6C 00 05 40

40 hex ->  0100 0000 bin

NAL unit of type PPS
Length: 4 bytes

00 00 00 04
34 00 FB 00

In the last byte of both cases, the RBSP stop bit should be set to 1.
In the case of SPS, RBSP has a length of 21 bytes, of which the data occupies 166 bits, leaving 2 bits (trailing bits).
The first of these is the RBSP stop bit and should have a value of 1.

In the case of PPS, the entire last byte of RBSP is filled with zeros.

Decoding works because the parsing functions for NAL units, such as xevd_eco_sps, xevdm_eco_sps, xevd_eco_pps, and xevdm_eco_pps, do not detect the RBSP stop bit.

According to the ISO/IEC 23094-1:2020 documentation,
RBSP stop bit is a bit equal to 1 present within a raw byte sequence payload (3.72) after a string of data bits (3.85), for which the location of the end within an RBSP (3.72) can be identified by searching from the end of the RBSP (3.72) for the RBSP stop bit, which is the last non-zero bit in the RBSP (3.72)

The last lines of the functions int xevd_eco_sps(XEVD_BSR *bs, XEVD_SPS *sps) and int xevdm_eco_sps(XEVD_BSR *bs, XEVD_SPS *sps) (files xevdm_eco.c and xevd_eco.c) look like this:

while (!XEVD_BSR_IS_BYTE_ALIGN(bs))
{
    xevd_bsr_read1(bs, &t0);
}

It seems that there is no detection of the RBSP stop bit here.

Meanwhile, according to the ISO/IEC 23094-1:2020 documentation, the first bit of the trailing bits should have a value of 1.

7.3.2.8 RBSP trailing bits syntax

rbsp_trailing_bits( ) {
rbsp_stop_one_bit  /* equal to 1 /
while( !byte_aligned( ) )
rbsp_alignment_zero_bit  / equal to 0 */
}

1. It seems to me that first, during encoding, the RBSP stop bit should be written after the SODB.

2. Second, during decoding, it should be checked whether the RBSP stop bit is set to 1 after the SODB.

Correct me please if I'm wrong.

[mss-park]

'rbsp_stop_one_bit' has been removed in updated version of ISO/IEC 23094-1:2020/Amd 1:2023(https://www.iso.org/standard/85138.html).
Please check it and let me know if there is any problems regarding this issue.