[Discuss-gnuradio] PFB mapping: M < N?

[John Ackermann N8UR]

I find the documentation about PFB channel mapping to be confusing. 
I've extracted it below.

The first paragraph seems to say the channel map has to have as many 
elements "M" as the number of outputs "N".

But the second and third paragraphs talk about cases where M < N, and 
only "M out of N channels are driven to an output stream."  This seems 
to contradict the first paragraph.

GRC allows me to make a channel map with M elements where M < N, but it 
still requires me to connect all N ports to a sink.  If that's so, why 
would one ever want map less than N elements?  There seems to be no 
benefit to doing so.

Is this documentation contradictory, or confusing, or am I missing 
something?

Thanks,
John

From https://gnuradio.org/doc/sphinx-3.7.0/filter/channelizers_blk.html:
"
The map should have the same number of elements as the number of output 
connections from the block. The minimum value of the map is 0 (for the 
0th channel) and the maximum number is N-1 where N is the number of 
channels.

We specify M as the number of output connections made where M <= N, so 
only M out of N channels are driven to an output stream. The number of 
items in the channel map should be at least M long. If there are more 
channels specified, any value in the map over M-1 will be ignored. If 
the size of the map is less than M the behavior is unknown (we don’t 
wish to check every entry into the work function).

This means that if the channelizer is splitting the signal up into N 
channels but only M channels are specified in the map (where M <= N), 
then M output streams must be connected and the map and the channel 
numbers used must be less than N-1. Output channel number can be reused, 
too. By default, the map is [0...M-1] with M = N.
"