CMX138 CML Microcircuits, CMX138 Datasheet - Page 34

• Manufacturer Part Number: CMX138
• Description: Audio Scrambler and Sub-Audio Signalling Processor
• Manufacturer: CML Microcircuits

Audio Scrambler and Sub-Audio Signalling Processor
CMX138
7.10 GPIO
Two pins on the CMX138 are provided for Rx and Tx Enables. These pins become active low when the
device enters the appropriate mode. These can be used for driving external circuitry and have the
advantage of having minimal delay from the activation of the selected mode and so are not dependant
upon any delays due to the transfer of commands / data over the C-BUS.
7.11 Signal Level Optimisation
The internal signal processing of the CMX138 will operate with wide dynamic range and low distortion
only if the signal level at all stages in the signal processing chain is kept within the recommended limits.
For a device working from a 3.3V ±10% supply, the maximum signal level which can be accommodated
without distortion is [(3.3 x 90%) – (2 x 0.3V)] Volts pk-pk = 838mV rms, assuming a sine wave signal.
Compared to the reference level of 308mV rms, this is a signal of +8.69dB. This should not be exceeded
at any stage.
7.11.1 Transmit Path Levels
For the maximum undistorted signal out of the MOD attenuator, the signal level at the output of the
Analogue block should not exceed +8.69dB, assuming both fine and coarse output attenuators are set to
a gain of 0dB. The sub-audio level is normally set to 31mV rms ±1.0dB, which means that the output from
the soft limiter must not exceed 803mV rms. If pre-emphasis is used, an output signal at 3000Hz will have
three times the amplitude of a signal at 1000Hz, so the signal level before pre-emphasis should not
exceed 268mV rms. If the compressor is also used, its ‘knee’ is at 100mV rms, which would allow a signal
into the compressor of 718mV rms, which is less than the maximum signal level. The Fine Input Gain
adjustment has a maximum attenuation of 3.5dB and no gain, whereas the Coarse Input Gain adjustment
has a variable gain of up to +22.4dB and no attenuation. If the highest gain setting were used, then the
maximum allowable input signal level at the MicFB pin would be 54mV rms. With the lowest gain setting
(0dB), the maximum allowable input signal level at the MicFB pin would be 718mV rms.
In some applications where there is a requirement for the system to operate with a significant overload on
the MIC input (+20dB) an external limiter may be required to ensure that the signal input does not exceed
the recommended CMX138 input levels. This can result in significant harmonic content (above 6kHz) that
should be removed by suitable input filtering.
7.11.2 Receive Path Levels
For the maximum undistorted signal out of the AUDIO attenuator, the signal level at the output of the
Analogue Routing block should not exceed +8.69dB, assuming both fine and coarse output attenuators
are set to a gain of 0dB. In this case, there is no sub-audio signal to be added, so the maximum signal
level remains at 838mV rms. If de-emphasis is used, an output signal at 300Hz will have three and a third
times the amplitude of a signal at 1000Hz, so the signal level before de-emphasis should not exceed
251mV rms. If the expander is also used, its ‘knee’ is at 100mV rms, which would allow a signal into the
expander of 158mV rms. The Fine Input Gain adjustment has a maximum attenuation of 3.5dB and no
gain, whereas the Coarse Input Gain adjustment has a variable gain of up to +22.4dB and no attenuation.
If the highest gain setting were used, then the maximum allowable input signal level at the DiscFB pin
would be 12.0mV rms. With the lowest gain setting (0dB), the maximum allowable input signal level at the
DiscFB pin would be 158mV rms. The signal level of +8.69dB (838mV rms) is an absolute maximum,
which should not be exceeded anywhere in the signal processing chain if severe distortion is to be
avoided.
© 2008 CML Microsystems Plc
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D/138_FI1.0/5