CP3SP33SMSSLASHNOPB datasheet(255/407 Pages) TI1

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CP3SP33

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SNOSCW5 – MAY 2013

26.2 PCM Conversions

During conversion between CVSD and PCM, any PCM format changes are done automatically depending

on whether the PCM data is µ-Law, A-Law, or linear. In addition to this, a separate function can be used to

convert between the various PCM formats as required. Conversion is performed by setting up the control

bit CVCTRLn.PCMCONV to define the conversion and then writing to the LOGINn and LINEARINn

registers and reading from the LOGOUTn and LINEAROUTn registers. There is no delay in the

conversion operation, and it does not have to operate at a fixed rate. It will only convert between µ-Law/A-

Law and linear, not directly between µ-Law and A-Law. (This could easily be performed by converting

between µ-Law and linear and between linear and A-Law.)

If a conversion is performed between linear and µ-Law log PCM data, the linear PCM data are treated in

the left-aligned 14-bit linear data format with the two LSBs unused. If a conversion is performed between

linear and A-Law log PCM data, the linear PCM data are treated in the left-aligned 13-bit linear data

format with the three LSBs unused.

If the module is only used for PCM conversions, the CVSD clock can be disabled by clearing the CVSD

Clock Enable bit (CLKEN) in the control register.

26.3 CVSD Conversion

The CVSD/PCM converter module transforms either 8-bit logarithmic or 13-bit to 16-bit linear PCM

samples at a fixed rate of 8 ksps. The CVSD to PCM conversion format must be specified by the

CVSDCONV control bits in the CVSD Control register (CVCTRLn).

The CVSD algorithm is designed for 2’s complement 16-bit data and is tuned for best performance with

typical voice data. Mild distortion will occur for peak signals greater than -6 dB. The Bluetooth CVSD

standard is designed for best performance with typical voice signals: nominally –6 dB with occasional

peaks to 0 dB rather than full-scale inputs. Distortion of signals greater than –6 dB is not considered

detrimental to subjective quality tests for voice-band applications and allows for greater clarity for signals

below –6 dB. The gain of the input device should be tuned with this in mind.

If required, the RESOLUTION field of the CVCTRLn register can be used to optimize the level of the 16-bit

linear input data by providing attenuations (right-shifts with sign extension) of 1, 2, or 3 bits.

Log data is always 8 bit, but to perform the CVSD conversion, the log data is first converted to 16-bit 2’s

complement linear data. A-law and µ-law conversion can also slightly affect the optimum gain of the input

data. The CVCTRL.RESOLUTION field can be used to attenuate the data if required.

If the resolution is not set properly, the audio signal may be clipped or attenuated.

26.4 Fixed-Rate PCM-to-CVSD Conversion

The converter core reads the double-buffered PCMIN register every 125 µs and writes a new 16-bit CVSD

data stream into the CVSD Out FIFO every 250 µs. If the PCMIN buffer has not been updated with a new

PCM sample between two reads from the CVSD core, the old PCM data is used again to maintain a fixed

conversion rate. Once a new 16-bit CVSD data stream has been calculated, it is copied into the 8 × 16-bit

wide CVSD Out FIFO.

If there are only three empty words (16-bit) left in the FIFO, the nearly full bit (CVNF) is set, and, if

enabled (CVSDINT = 1), an interrupt request is asserted.

If the CVSD Out FIFO is full, the full bit (CVF) is set, and, if enabled (CVSDERRINT = 1), an interrupt

request is asserted. In this case, the CVSD Out FIFO remains unchanged.

Within the interrupt handler, the CPU can read out the new CVSD data. If the CPU reads from an already

empty CVSD Out FIFO, a lockup of the FIFO logic may occur which persists until the next reset. Software

must check the CVOUTST field of the CVSTAT register to read the number of valid words in the FIFO.

Software must not use the CVNF bit as an indication of the number of valid words in the FIFO.

Dual CVSD/PCM Conversion Modules

255

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