CMX264D5 データシートの表示（PDF） - CML Microcircuits

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CMX264D5

Frequency Domain Split Band Scrambler

CML Microcircuits 

Frequency Domain Split Band Scrambler

CMX264

1.5.1.6 Upperband Balanced Modulator

This modulates the output of the Tx Upperband Input Filter (TXUBIPF) to form two sidebands, a

frequency inverted lower sideband and a non-inverted upper sideband. Part of the inverted lower

sideband is selected by the Tx Upperband Output Filter (TXUPOPF) to form the transmitted

upperband, i.e. that part of the audio spectrum above the split point.

For each split point setting, the upper carrier frequency is chosen so that baseband frequencies close

to 2770Hz are shifted to just above the split point. Conversely, baseband frequencies just above the

split point are shifted to the upper transmission limit of the channel. The result is that the upper carrier

is the same (approximate) distance above the lower carrier at all split point settings. The

approximation arises because of the need to have frequencies derivable from the crystal frequency

and the fact that only certain divisors of associated filter sample rates have been chosen as carriers, in

order to avoid aliasing.

1.5.1.7 Tx Upperband Output Filter (TXUBOPF)

A lowpass filter whose input is the output signal from the upperband balanced modulator. Its function

is to select that part of the frequency inverted lower sideband which will form the upperband part of the

transmitted scrambled audio. This output is summed with the output of the lowerband Tx channel to

form the complete scrambled signal for transmission.

This filter's response, together with any external filtering e.g. a lowpass filter used to ensure

compliance with statutory transmission requirements, defines the upper frequency of the transmitted

scrambled signal.

1.5.1.8 Tx Summer

This sums the lowerband and upperband to form the complete scrambled audio signal prior to

transmission.

1.5.1.9 Tx Channel De-Emphasis

A de-emphasis circuit at the output of the Tx scramble block. It has a slope of -6dB per octave

between 280Hz and 3140Hz. It may be used to compensate for pre-emphasis in the transmission

channel.

1.5.2 Rx Channel

1.5.2.1 Rx Channel Pre-Emphasis

A pre-emphasis circuit at the input of the Rx descramble block. It has a slope of 6dB per octave

between 280Hz and 3140Hz. In descramble mode, it may be selected to pre-emphasise the received

audio signal prior to signal recovery. It may, for example, be used to compensate for de-emphasis in

the transmission channel.

1.5.2.2 Rx Lowerband Input Filter (RXLBIPF)

A lowpass filter whose function is to select the lowerband part of the received scrambled signal. Its

bandwidth is controlled by the split point setting, which should be chosen to match the spectrum of the

received lowerband.

1.5.2.3 Rx Lowerband Balanced Modulator

This modulates the output of the preceding Rx Lowerband Input Filter (RXLBIPF). The carrier is

identical to that of the lowerband carrier in the transmitter section. The output consists of two

sidebands.

The lower sideband is inverted with respect to the received lowerband, i.e. returned to its original

frequency with respect to the original baseband signal. If the Tx and Rx carriers are the same

frequency, the lower sideband is the desired recovered lowerband.

The upper sideband is not required in the final descrambled output. It is removed by the Rx

Lowerband Output Filter (RXLBOPF)

© 1999 Consumer Microcircuits Limited

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D/CMX264/1

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