TNS_MAX_ORDER is object type and short/long window dependent

[atimmernans]

I was looking at some AAC specs and they mention that TNS_MAX_ORDER is a constant that is dependent of the object type and can takes values 12 or 20 for long windows and 7 for short windows. FAAD2 seems to only use value 20.

Since they are some draft specs found on the net and not official ones can you check this point?

[eustas]

For short window it is capped with 7 indeed (search for order_bits = 3 and tns_order = min(tns->order[w][f], TNS_MAX_ORDER).
For long window it is capped with 20, but again it could be 0..20.

[eustas]

@atimmernans it would be nice if you provide some quotes, to be sure we have coded it right. Currently I do not have access to spec to check it myself.

[atimmernans]

From that ISO/IEC 14496-3:2009 document:

4.6.9.4 Maximum TNS order and bandwidth
The value for the constant MAX_TNS_ODER depends on audio object type and windowing, and. Table 4.156
defines MAX_TNS_ORDER depending on these parameters.
Table 4.156 – Definition of TNS_MAX_ORDER depending on AOT and windowing
windowing short windows long windows
AOT 1 (AAC Main) 7 20
other AOT using TNS 7 12