Re: How does MPE handle the last byte?

[Dr G Fairhurst <gorry@erg.abdn.ac.uk>]

OK, seems clear to me.

So, if I understand this, it means:

The receiver stops processing the MPEG-2 TS Packet
Payload after it has processed the end of the MPE Section, 
when there is no PUSI set. 

That is, it will silently discard the last (unused)
byte in the example below?

Gorry

Patrick Cipiere wrote:
> 
> > What does the encapsulator do if this TS-Packet has no PUSI
> > set, since setting the PUSI flag implies that encapsulator
> > would also include a Payload start pointer in the first
> > position after the TS-Packet header. This pointer consumes
> > the last byte... leaving nothing for it to point at.
> 
> >From ISO/IEC 13818-1
> --------------------
> 2.4.4.2 Semantics definition of fields in pointer syntax pointer_field
> This is an 8-bit field whose value shall be the number of bytes,
> immediately following the pointer_field until the first byte of the
> first section that is present in the payload of the Transport Stream
> packet (so a value of 0x00 in the pointer_field indicates that the
> section starts immediately after the pointer_field). When at least one
> section begins in a given Transport Stream packet, then the
> payload_unit_start_indicator (refer to 2.4.3.2) shall be set to 1 and
> the first byte of the payload of that Transport Stream packet shall
> contain the pointer. When no section begins in a given Transport
> Stream packet, then the payload_unit_start_indicator shall be set to 0
> and no pointer shall be sent in the payload of that packet.
> 
> My understanding, and my implementation:
> 
> 1)if we have only 1 byte left, and no PUSI bit already set, i do not
>   think we can do anything with this byte.
>   This even can't be ISO/IEC 13818-1 conformant, because we have to
>   shift the current payload by one byte, set the PUSI bit to one and
>   set the pointer_field to 183, which points outside of the transport
>   packet paylaod ...
> 
> 2)if we have only 2 bytes left, and no PUSI bit already set, we could
>   use these 2 bytes.
>   Shift the the current payload by one byte, set the PUSI bit to one and
>   set the pointer_field to 182, and have the first byte of the new
>   payload unit in the last byte of the transport packet.
>   Then is is a matter of space vs cpu decision: move 182 bytes to save
>   1 byte for the bandwidth's sake.
>   This will depend on the cpu cycles available on the encapsulator.
> 
>   Anyway, the receiver/decoder must be able to deal with such
>   situations.
>   1) is an error: pointer outside of the transport packet
>   2) is OK
> 
> Patrick.
> --
> UDcast: Full IP over Broadcast Media
> 
> Phone:  (+33) (0)4 93 00 16 99
> Mobile: (+33) (0)6 14 21 55 98
> Fax:    (+33) (0)4 93 00 16 61                 http://www.UDcast.com