[M4IF Technotes] Descriptor Length Field

[Trevor]

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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