4.2 Voice Encoding Formats

(Page 3 of 8 in this chapter)

When recording or playing speech files, the application must select an encoding format. The primary issue to consider when selecting a format is the compression ratio vs. fidelity. More aggressive compression requires less disk space and reduces host-to-AG loading.
On heavily loaded systems, the throughput requirements between the host and the AG board may cause gaps in the speech record/playback. This is called an "underrun condition." Greater compression may be necessary in this case to eliminate the problem.
The AG Access library supports multiple file encoding formats which are listed in the table below. The encoding is the suffix for ADI_ENCODE_ names used when writing 'C' code. For example, ADI_ENCODE_NMS_24 is the NMS_24 encoding. ADI_ENCODE extension Description NMS_16 NMS ADPCM 16kbs NMS_24 NMS ADPCM 24kbs NMS_32 NMS ADPCM 32kbs NMS_64 framed PCM 64kbs MULAW mu-law 64kbs ALAW A-law 64kbs PCM8M16 PCM 8k mono 16 bit OKI_24 OKI ADPCM 24kbs OKI_32 OKI ADPCM 32kbs PCM11M8 PCM 11kHz 8 bit (Wave format) PCM11M16 PCM 11kHz 16 bit (Wave format) G726 G726 ADPCM 32kbs
The AG board processes speech in atomic units called voice frames. Frames vary in size and duration depending upon the encoding format. The following table specifies physical constants for each encoding format. ADI_ENCODE_ Extension Sample Size (bits) Sample Rate (Hz) Frame Size (Bytes) Frame Duration (ms) Throughput (Bytes/sec) NMS_16 2 8000 42 20 2100 NMS_24 3 8000 62 20 3100 NMS_32 4 8000 82 20 4100 NMS_64 8 8000 162 20 8100 MULAW 8 8000 80 10 8000 ALAW 8 8000 80 10 8000 PCM8M16 16 8000 160 10 16000 OKI_24 4 6000 30 10 3000 OKI_32 4 8000 40 10 4000 PCM11M8 8 11000 110 10 11000 PCM11M16 16 11000 220 10 22000 G726 4 8000 40 10 4000

`ADI_ENCODE` extension	Description
`NMS_16`	NMS ADPCM 16kbs
`NMS_24`	NMS ADPCM 24kbs
`NMS_32`	NMS ADPCM 32kbs
`NMS_64`	framed PCM 64kbs
`MULAW`	mu-law 64kbs
`ALAW`	A-law 64kbs
`PCM8M16`	PCM 8k mono 16 bit
`OKI_24`	OKI ADPCM 24kbs
`OKI_32`	OKI ADPCM 32kbs
`PCM11M8`	PCM 11kHz 8 bit (Wave format)
`PCM11M16`	PCM 11kHz 16 bit (Wave format)
`G726`	G726 ADPCM 32kbs

ADI_ENCODE_ Extension	Sample Size (bits)	Sample Rate (Hz)	Frame Size (Bytes)	Frame Duration (ms)	Throughput (Bytes/sec)
`NMS_16`	2	8000	42	20	2100
`NMS_24`	3	8000	62	20	3100
`NMS_32`	4	8000	82	20	4100
`NMS_64`	8	8000	162	20	8100
`MULAW`	8	8000	80	10	8000
`ALAW`	8	8000	80	10	8000
`PCM8M16`	16	8000	160	10	16000
`OKI_24`	4	6000	30	10	3000
`OKI_32`	4	8000	40	10	4000
`PCM11M8`	8	11000	110	10	11000
`PCM11M16`	16	11000	220	10	22000
`G726`	4	8000	40	10	4000

4.2.1 Buffer Sizes

All play buffers submitted to AG Access must be an integral number of frames for the selected encoding format. For example, if you select ADI_ENCODE_NMS_24, the play buffer size must be a multiple of 62 bytes. Failure to submit a play buffer meeting this size requirement causes the function to terminate with an ADIERR_BAD_SIZE reason. Other than the multiple frame size requirement, buffers submitted to AG Access can be of arbitrary size.
Each AG board has a physical buffer size which is both board and encoding dependent. If you submit a buffer larger than the physical size, AG Access divides the buffer into physical segments and submits those segments to the AG board. To eliminate fractional buffers and to reduce the AG-to-host interactions, the optimum user buffer will be a multiple of the physical AG buffer size. This size is retrieved via the function adiGetEncodingInfo.

(Page 3 of 8 in this chapter)