[M4IF Technotes] Descriptor Length Field

[Chandra Sekhar Reddy]

Hi Trevor,
This field is not of fixed length.
If the length is fixed, smaller-size-descriptors will be wasting bits and also
expandability is a problem.
This way the size has no limits.
It is not wasting bits, infact saving bits.
Regards,
Chandra
Trevor wrote:
> Hi Chandra,
>
> I see. It is due to the "expandable"!
> Thanks Chandra. But I am just curious why it is encoded in this way.
> Isn't it wasting bits?
>
> Rgds,
> Trevor
>
> Chandra Sekhar Reddy wrote:
>
> >Hi Trevor,
> >
> >The Descriptors are derived from *BaseDescriptor*
> >These Descriptors
> >--start with their identifier (the descriptor-tag 1-byte field)
> >--followed by *sizeOfInstance* (the length-field in question!)
> >
> >This sizeOfInstance gives the total-number of bytes occupied by the
> >descriptor,
> >*excluding* the tag-field and sizeOfInstance-field.
> >
> >Refer to "14.3.3 Expandable classes" -- read the meaning of sizeOfInstance.
> >******************************
> >int sizeOfInstance = 0;
> >bit(1) nextByte;
> >bit(7) sizeOfInstance;
> >while(nextByte) {
> >    bit(1) nextByte;
> >    bit(7) sizeByte;
> >    sizeOfInstance = sizeOfInstance<<7 | sizeByte;
> >}
> >******************************
> >
> >Regards,
> >Chandra
> >
> >
> >
> >Trevor wrote:
> >
> >
> >
> >>Dear Experts,
> >>
> >>According to "ISO/IEC 14496-1:2001", the syntax of any descriptor does
> >>NOT contain a length field. But in the annex J (Graphical representation
> >>of osbject descriptor and sync layer syntax), there is a length field
> >>right after TAG field. Could anyone here tell me what this length field
> >>represents and how it is encoded?
> >>
> >>Thanks in advance,
> >>Trevor
> >>
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