Writing a client of BASS¶

This section provides information about designing clients of the BASS component. It first defines a formal algorithm that clients could use, and shows a sample implementation in a GenoM3 component called BASC.

An algorithm for clients of BASS¶

A client of BASS can face many different situations:

• It may need just a single block of data (e.g. we could think of a client requesting 2 seconds of audio data to learn noise properties), and the block may be longer than the total size of the port.
• It may indefinitely request new blocks of data, with the requirement that they follow each other without frame loss between two consecutive blocks.
• It may request data faster than the port update rate. Or conversely, it may not read the port often enough, leading to data loss. And in case of data loss, it must know how many frames were lost.

Let us define $$nFOP$$ as the total number of frames on the port. The data structure used on the Audio port, of type portStruct, is recalled in Table 3.

Table 3 Formal definition of the data type portStruct used on the Audio port of BASS
Field Data type Comment
sampleRate unsigned integer sample rate in Hz
nChunksOnPort unsigned integer number of chunks on the port ($$nCOP$$)
nFramesPerChunk unsigned integer number of frames per chunk ($$nFPC$$)
lastFrameIndex unsigned integer index for tracking data
left[nFOP] array of integers arrays of $$nFOP$$ samples, with $$nFOP = nFPC * nCOP$$
right[nFOP] array of integers

The left and right fields are arrays of $$nFOP$$ samples each, updated as FIFOs (c.f. related section in BASS documentation). What the client need is to copy blocks of given size $$N$$ from these arrays. Let us define a function getAudioData, taking $$N$$ as input and returning one copied block.

In order not to miss any frame between blocks retrieved with two consecutive calls, the getAudioData function also takes both as input and output an index of the next frame to be read, noted $$nfr$$. For instance, the client get a first block of $$N$$ frames starting from a given index $$nfr$$. The function must return, along with the block of data, the new value of the index, corresponding to the next frame right after the first retrieved block. Then, the client can call the function again with this new value for $$nfr$$, so as to get the second block starting from this point.

For the very first block, the client can choose $$nfr$$ according to the current value of the lastFrameIndex.

• If it wants to pick data from the existing frames on the port, $$nfr$$ is chosen to be less than lastFrameIndex.
• If it wants to get fresher data (frames that are not yet on the port but will be published shortly on it), $$nfr$$ is chosen to be greater than lastFrameIndex.

With a call to getAudioData, the client requests a block of given size $$N$$, but the function may not be able to return a full block of $$N$$ frames. Indeed, the ending point for the desired block may be a frame that is not yet published by the server. So, the function returns the number $$n$$ of frames it can get ($$n \le N$$). The client can then call the function again and ask for the remaining frames in a loop until the requested block is complete. Here are examples of when this can occur:

• The client requests data more often than they are captured by the microphones (which should be the regular case, because a slower client will end up losing data).
• The client requests a block which is longer than the total number of frames on the port ($$N > nFOP$$).
• At first call, if the client sets $$nfr$$ to a greater value than lastFrameIndex, then getAudioData will not return any available frame ($$n = 0$$). But the client can keep calling the function in a loop until it gets the requested frames.

Finally, in case of data loss, getAudioData also returns the number of frames that were lost. The retrieved block then starts at the first frame that is still available on the port (the oldest one).

Below is the getAudioData function written with formal syntax:

function: getAudioData
|
| inputs:  integer     N     (number of frames the client wants to get)
|          portStruct  Audio (data from the the output port of bass)
|
| outputs: integer     n     (number of frames the function was able to get)
|          integer     loss  (number of lost frames, 0 if no loss)
|          array(int)  l[n]  (the retrieved data block from left channel)
|          array(int)  r[n]  (the retrieved data block from right channel)
|
| in&out:  integer     nfr   (index of the Next Frame to Read)
|
| local:   integer     nFOP  (total number of Frames On the Port)
|          integer     lfi   (Index of the Last Frame on the port)
|          integer     ofi   (Index of the Oldest Frame on the port)
|          integer     pos   (current position in the left and right arrays)
|
| algorithm
|  |
|  | nFOP <- Audio.nFramesPerChunk * Audio.nChunksOnPort
|  | lfi <- Audio.lastFrameIndex
|  | ofi <- max(0, lfi - nFOP + 1) //if the acquisition just started and the
|  |                               //port is not full yet, ofi equals 0
|  | /* Detect a data loss */
|  | loss <- 0
|  | if (nfr < ofi)
|  |  | loss <- ofi - nfr
|  |  | nfr <- ofi
|  | end if
|  |
|  | /* Compute the starting position in the left and right input arrays */
|  | pos <- nFOP - (lfi - nfr + 1)
|  |
|  | /* Fill the output arrays l and r */
|  | n <- 0
|  | while (n < N AND pos < nFOP)
|  |  | l[n] <- Audio.left[pos]
|  |  | r[n] <- Audio.right[pos]
|  |  | n <- n + 1
|  |  | pos <- pos + 1
|  |  | nfr <- nfr + 1
|  | end while
|  |
|  | return (l[], r[], n, nfr, loss)
|  |
| end algorithm
|
end function


Sample implementation in a GenoM3 component¶

BASC is a GenoM3 component that acts as a client of BASS. It can be connected to the output port of BASS, and implements the above generic algorithm to retrieve blocks of audio signals. BASC does not perform any processing on the data. It only shows what a GenoM3 client of BASS could look like.

The folder containing source files of the component is named basc-genom3 and is located in the RoboticPlatform folder of the software repository. All files that are referred to in this section are in the basc-genom3 folder.

Services¶

BASC runs the above algorithm in a service called GetBlocks, defined in description file basc.gen. It expects the following input parameters:

Name Data type Default value Documentation
nBlocks unsigned long 1 Amount of blocks, 0 for unlimited
nFramesPerBlock unsigned long 12000 Block size in frames
startOffs long -12000 Starting offset (past < 0, future > 0)

The first parameter nBlocks sets the number of blocks that the service must get, or can be set to 0 to run the service indefinitely. The second parameter nFramesPerBlock sets the block size ($$N$$ in the previous section). Last, the startOffs parameter sets the index of the first frame to read, relatively to the current value of the Last Frame Index on the port. For instance, if startOffs is -1000 and lastFrameIndex is 43000 when the service is called, then the first frame to read ($$nfr$$ in previous section) will have index 42000. With the given default values, GetBlocks will get 1 block of 12000 frames, taking the last 12000 frames on the port.

At any moment, the GetBlocks service can be interrupted by calling a service named Stop.

Execution example¶

Assume that the GetBlocks service runs at a period of, say, 250ms (defined in the dotgen file). So every 250ms, it calls the getAudioData function, either to request a new block or to complete the current block if the previous call could not return a full one. Depending on the sample rate, if the requested block size (parameter nFramesPerBlock) is too small so that one block is less than 250ms, then the client will eventually loose some frames. On the other hand, if a block lasts more than 250ms, then the client will request data at a rate higher than their update frequency, which should be fine.

This can be tested: assume that the sampling rate is 44100Hz. So, 250ms is 11025 frames. Calling GetBlocks with nFramesPerBlock < 11025 leads to data loss. Following is an example with the matlab-genomix client.

% The middleware, genomix, bass and basc should be running

% Connect to genomix
>> client = genomix.client

% Load the server BASS and the client BASC

% Connect the input port of BASC to the output port of BASS
>> basc.connect_port('Audio', 'bass/Audio')

% Start the acquisition (using default values)
rAcquire = bass.Acquire('-a', 'hw:1,0', 44100, 2205, 20)

%%% EXAMPLE 1: get one block of 2 seconds (88200 frames at 44100Hz)
>> basc.GetBlocks(1, 88200, 0)

% In the terminal where it runs, BASC prints:
%
%    Requested  88200 frames, got  11025.
%    Requested  77175 frames, got  11025.
%    Requested  66150 frames, got  11025.
%    Requested  55125 frames, got  11025.
%    Requested  44100 frames, got  11025.
%    Requested  33075 frames, got  11025.
%    Requested  22050 frames, got  11025.
%    Requested  11025 frames, got  11025.
%    A new block is ready to be processed.
%
% Each line 'Requested N frames, got n.' indicates the number N of frames
% requested by BASC, and the number n it got in return. The component keeps
% requesting frames until it has formed a block of 88200 frames.

%%% EXAMPLE 2: get unlimited number of blocks, with nFramesPerBlock < 11025
>> rGetBlocks = basc.GetBlocks('-a', 0, 10000, 0)

% After a few seconds, BASC prints:
%
%    Requested  10000 frames, got  10000.
%    !!Lost 1025 frames!!
%    A new block is ready to be processed.
%    Requested  10000 frames, got  10000.
%    !!Lost 1025 frames!!
%    A new block is ready to be processed.
%
% At each attempt to get the following block, some frames are lost because
% BASC does not read the port of BASS often enough.

% Stop the running GetBlocks service
>> basc.Stop()

%%% EXAMPLE 3: get unlimited number of blocks, with nFramesPerBlock > 11025
>> rGetBlocks = basc.GetBlocks('-a', 0, 12000,0)

% Here, as BASC reads the port slighlty faster than its update rate, the
% retrieved block is sometimes incomplete, for instance:
%
%    Requested  12000 frames, got  12000.
%    A new block is ready to be processed.
%    Requested  12000 frames, got  12000.
%    A new block is ready to be processed.
%    Requested  12000 frames, got  11550.
%    Requested    450 frames, got    450.
%    A new block is ready to be processed.
%    Requested  12000 frames, got  12000.
%    A new block is ready to be processed.
%
% The two consecutive lines 'Requested...' show that a first call only get
% 11550 frames, so the component  makes a second request to get the remaining
% part of 12000 - 11550 = 450 frames.

% Stop the running GetBlocks service
>> basc.Stop()

% Kill the components
>> bass.kill()
>> basc.kill()

% Remove the used objects in Matlab
>> delete(bass)
>> delete(basc)
>> delete(client)

% The remaining processes (the middleware and genomix) can be killed


The getAudioData function in charge of getting the requested block is written in C in file codels/basc_read_codels.c. The codels of the GetBlocks service are also written in this file, with comments to explain the overall process followed by BASC.