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
8
D/CMX264/1
CMLMICRO [ CML MICROCIRCUITS ]
