Voice decoder channel filter

Decodes voice data according to a particular algorithm. Parameters within the decoder filter, such as payload ID, IP format and gain, can be altered through filter commands. Decoder filter status information can also be obtained through filter queries. For information about voice encoder filter parameters, refer to Voice channel parameter structures.

Filter ID

The following table lists some of the Fusion decoder filters:

Filter ID	Description	Frame size
MSP_FILTER_G711_DECODER	G.711 64 kbit/s mu-law or A-law decoder	10 ms
MSP_FILTER_G723_DECODER	G.723.1/A 6.4/5.3 kbit/s decoder	30 ms
MSP_FILTER_G726_DECODER	G.726 32 kbit/s decoder	10 ms
MSP_FILTER_G729_DECODER	G.729A/B 8.0 kbit/s decoder	10 ms

Initialization structure

MSP_VOICE_CHANNEL_PARMS

Restrictions

Used in the full duplex voice or simplex voice decoder channels.

Filter connections

Allowed connections for voice decoder filters include:

Input	Output
MSP_FILTER_JITTER	MSP_ENDPOINT_DS0

Commands

Enabled voice decoder filters respond to the following commands when the associated channel is enabled:

Command ID	Description
MSP_CMD_DECODE_MODE	Vocoding mode. Off-line means that the decoder filter plays silence to the DS0 endpoint. 0x0000: off line 0x0001: online Default is online.
MSP_CMD_DECODE_DTMF_MODE	Specifies if and how the decoder filter plays DTMF tones when it receives DTMF data (only when inband DTMF carriage is enabled). The application concatenates a control value and a playout value to determine how this works. Defined values for control include: DTMF_DISABLED MSPP channel does not play a DTMF tone when it receives an RFC 2833 compliant inband DTMF carriage packet. The following values can be ORed to specify decoder functionality: DTMF_INBAND_ENABLED MSPP channel automatically plays a DTMF tone when it detects an RFC 2833 compliant inband DTMF carriage packet. This setting is enabled by default. DTMF_PLAY_ENABLED MSPP channel plays DTMF tones when the application sends an MSP_CMD_DECODE_PLAY_DTMF_DIGIT command. The playout value is an integer that specifies number of decoder frames for an ongoing tone to continue when no end-of-tone packet is received.
MSP_CMD_DECODE_PLAY_DTMF_DIGIT	Commands the voice decoder filter to play a specified DTMF digit.
MSP_CMD_DECODE_GAIN	Gain value applied to the decoder filter output in S5.10 format. Default is 0x0400 (0 dB). Refer to Calculating vocoder filter gain and loss for a mapping of decibel gain/loss values to linear gain/loss values.
MSP_CMD_DECODE_IP_FORMAT	Specifies the companding algorithm applied to the IP payload (applies to G711 decoder only): 0x0000: mu-law (for T1 systems) 0x0001: A-law (for E1 systems) Default matches system settings.
MSP_CMD_DECODE_PAYLOAD_ID	Assigns a payload ID for incoming Fusion media frames. Valid range is 96 - 127. Refer to Vocoder filter attributes for a list of default payload IDs.
MSP_CMD_DECODE_SILENCE_CODE	Specifies the silence pattern sent out by the decoder: 0 – 0xFF (X-law independent) 1 – 0x10 (X-law independent) This command is supported only by the G.711 decoder. All other decoders respond with FUNCTION_NOT_IMPLEMENTED.
MSP_CMD_DECODE_ALL	Resets any or all of the decoder filter parameters.

Note: Voice decoder filters must be enabled when you send them commands.

Caution:

When you change the payload ID of the voice decoder filter, you must also change the payload ID to the same value for the associated RTP endpoint. The voice encoder at the sending gateway must also match this payload ID.

The following table summarizes the commands that available decoder filters support (x = supported):

Decoder	Mode	Gain	IP format	Payload ID	Query	Play DTMF	DTMF mode	Silence code
G.711	x	x	x	x	x	x	x	x
G.723.1	x	x		x	x	x	x
G.726	x	x	x	x	x	x	x
G.729A	x	x	x	x	x	x	x

When sending commands (except for MSP_CMD_DECODE_ALL commands) to decoder filters, include a pointer the following structure:

typedef struct tag_msp_FILTER_DECODE_CMD {
WORD value;
} msp_FILTER_DECODE_CMD;

Use the value field to specify a value for the specified command.

When sending MSP_CMD_DECODE_ALL decoder commands, applications provide a pointer to an msp_FILTER_DECODE_ALL_CMD structure. This structure contains all of the parameters used to configure the decoder filter when the application created the voice channel.

When using the MSP_CMD_DECODE_ALL command, applications can specify a value of 0xFFFF for 16-bit parameters or 0xFFFFFFFF for 32-bit parameters (or -1 in decimal format) to retain the existing settings for any voice decoder parameter. Otherwise the MSPP service expects a new value for every parameter in the msp_FILTER_DECODE_ALL_CMD structure. For more information about decoder filter parameters, refer to the mspcmd.h header file.

For a list of command IDs and their associated structures, refer to the mspinit.h header file.

Queries

Voice decoder filters respond to the following queries when the associated channel is enabled:

MSP_QRY_SILENCE_CODE
Returns the configured silence pattern for the decoder in an msp_FILTER_SILENCE_CODE structure.
This query is supported only by the G.711 decoder. All other decoders respond with FUNCTION_NOT_IMPLEMENTED.
Sending an MSP_QRY_SILENCE_CODE query returns the following structure:

typedef struct tag_msp_FILTER_SILENCE_CODE {
DWORD FilterId;
WORD CommandEcho;
WORD SilenceCode;
} msp_FILTER_SILENCE_CODE;

The returned query structure provides the following information:

Field	Description
FilterID	Filter ID of the specific decoder filter.
CommandEcho	Return of the query command (0x12).
SilenceCode	Decoder silence pattern (0xFF / 0x10).

MSP_QRY_DECODE_STATUS
Returns information about the decoder filter state in an msp_FILTER_DECODE_STATE structure.
Sending an MSP_QRY_DECODE_STATUS query returns the following structure:

typedef struct tag_msp_FILTER_DECODE_STATE{

   DWORD  FilterId;
   WORD   CommandEcho;
   WORD   State;
   WORD   Gain;
   WORD   Rate;
   WORD   PayloadID;
   WORD   IPFormat;
   WORD   DtmfMode;
   WORD   DSPId;
   WORD   ActiveCnt;
   WORD   SIDCnt;
   WORD   NullCnt;
   WORD   DtmfCnt;
   WORD   InvalidPktCnt;
   WORD   DSInQEmptyCnt;
   WORD   DSInQExtraCnt;

} msp_FILTER_DECODE_STATE;

The returned query structure provides the following information:

Field	Description
FilterID	Filter ID of the specific decoder filter.
CommandEcho	Return of the query command (0x10).
State	Decoder filter state (online/off line).
Gain	Decoder filter gain.
Rate	Decoder rate.
PayloadId	Expected payload ID of incoming data.
IpFormat	Expected IP format of incoming data (G.711 and G.726 only).
DtmfMode	Whether the voice decoder plays DTMF tones as a result of RFC 2833 compliant packets.
DSPId	DSP number where decoder filter resides.
ActiveCnt	Number of active data packets received.
SIDCnt	Number of SID data packets received.
NullCnt	Number of null data packets received.
DtmfCnt	Number of inband DTMF packets received.
InvalidPktCnt	Number of invalid data packets received.
DSInQEmptyCnt	Number of times the decoder filter received no data.
DSInQExtraCnt	Number of times the decoder filter received more than one data packet.

Unsolicited events

Voice decoder filters generate the following unsolicited event when the DtmfMode parameter is set:

Event	Description
MSPEVN_DPF_DTMF_PLAY_END	When the application commands the decoder to generate a DTMF tone (with the MSP_CMD_DECODE_PLAY_DTMF_DIGIT command), this event signals that the DTMF tone was generated.

Details

This filter accepts data from a jitter filter, decodes and expands the compressed data, and outputs the resultant PCM signal data to a DS0 endpoint.

Full-duplex voice channels combine voice encoder filters and voice decoder filters to create a two way flow of data. Simplex send channels use voice decoder filters, but without a matching voice encoder filter.

Encoder types and decoder types within a channel do not have to match. Applications can create duplex channels by creating pairs of simplex voice channels that use different vocoding algorithms. For more information about creating this kind of asymmetrical duplex connection, refer to Connecting simplex and duplex channels. Applications can load any mix of encoders and decoders on a CG board (within the constraints of CG board resource management).

The following illustration shows a decoder filter: