PCM - DECODING DECODING DECODING
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1. in the LSB position of each frame The search is made by examining a section of data whose length is a multiple of eight bits The length of this section can be changed by the on board switch SW3 Under noisy conditions it is advantageous to use longer lengths The switch settings are listed in Table A 1 below left toggle right toggle groups of eight bits o w s DOWN DOWN Copyright 2005 Emona Instruments Pty Ltd 4 42. The number of quantizing levels at the transmitter can be checked and their values 1 compare the quantizing levels just measured with those determined in the Lab Sheet entitled PCM encoding 2 reset the coding scheme on both modules to 7 bit Sweep the input DC signal over the complete range as before Notice the granularity in the output is almost un noticeable compared with the 4 bit case There are now 2 rather than 2 steps over the range a periodic message It was not possible when examining the PCM ENCODER in the Lab Sheet entitled PCM encoding to see the sample and hold waveform within the encoder But assuming perfect decoding it is available at the output of the decoder With a periodic message its appearance may be more familiar 1 change to a periodic message by connecting the SYNC MESSAGE of the PCM ENCODER via a BUFFER AMPLIFIER to its input Vin An amplitude of 2 Vpp is suitable Slow down the oscilloscope sweep speed to 1 ms cm Observe and record the signal at CH2 A When you agree that what you see is what you expected to see prepare to make a change and predict the outcome Currently the encoding scheme is generating a 4 bit digital word for each sample What would be the change to the waveform now displaying on CH2 A if at the encoder the coding scheme was changed from 4 bit to 7 bit Sketch your answer to this question show the waveform before and then after the change 2 chang
3. PCM DECODING modules advanced PCM DECODER PCM ENCODER preparation The signal to be decoded will be provided by the PCM ENCODER module as set up in the Lab Sheet entitled PCM encoding which should have already been completed Also read about the PCM DECODER module in the TIMS Advanced Modules User Manual frame synchronization Frame synchronization may be achieved either automatically embedded information in the received data or by stealing the FS signal from the transmitter See page 4 companding This is available but is not discussed in this Lab Sheet Read about it decoding The PCM DECODER module is driven by an external clock stolen from and so synchronized to that of the transmitter Upon reception the PCM DECODER 1 extracts a frame synchronization signal FS from the data itself from the embedded alternate ones and zeros in the LSB position or uses an FS signal stolen from the transmitter see above 2 extracts the binary number which is the coded and quantized amplitude of the sample from which it was derived from the frame identifies the quantization level which this number represents generates a voltage proportional to this amplitude level presents this voltage to the output Vouw The voltage appears at Vou for the duration of the frame under examination 6 message reconstruction can be achieved albeit with some distortion by lowpass filtering aA Uw encoding At t
4. e the coding scheme from 4 bit to 7 bit That is change the front panel toggle switch of both the PCM ENCODER and the PCM DECODER from 4 bit to 7 bit Observe record and explain the change to the waveform on CH2 A message reconstruction TIMS Lab Sheet It can be seen qualitatively that the output is related to the input The message could probably be recovered from this waveform But it would be difficult to predict with what accuracy Lowpass filtering of the waveform at the output of the decoder will reconstruct the message although theory shows that it will not be perfect It will improve with the number of quantizing levels If any distortion components are present they would most likely include harmonics of the message If these are to be measurable visible on the oscilloscope in the present case 2 the message should be set up to be a 130 Hz sinewave synchronized to the sampling rate Copyright 2005 Emona Instruments Pty Ltd 3 4 Emona TIMS PCM decoding L 25 rev 1 3 then they must not be removed by the filter and so give a false indication of performance See the Lab Sheet entitled Amplifier overload So we could look for harmonics in the output of the filter But we do not have conveniently available a spectrum analyzer An alternative is to use a two tone test message Changes to its shape especially its envelope are an indication of distortion and are more easily observed with an oscilloscope than w
5. he encoder the sample and hold operation before encoding is executed periodically It produces a rectangular pulse form Each pulse in the waveform is of exactly the same amplitude as the message at the sampling instant It is not possible to recover a distortionless message from these samples They are flat top rather than natural samples Call this the sampling distortion At the encoder the amplitude of this waveform was then quantized Itis still a rectangular pulsed waveform but the amplitude of each pulse will in general be in error by a small amount Call this waveform s t if the sample is held for as long as the sampling period it is a stepped waveform If the sample is held for a shorter time it is a rectangular waveform or pulseform It need only be held long enough for the quantizer to make its decision about which of the available quantized amplitudes to allocate to the sample Copyright 2005 Emona Instruments Pty Ltd J Lab Sheet vee 1 4 Emona TIMS PCM decoding L 25 rev 1 3 This is examined in the Lab Sheet entitled Sampling with SAMPLE amp HOLD to which reference should be made The voltage at Vow of the decoder is identical with s t above The decoder itself has introduced no distortion of the received signal But s t is already an inexact version of the sample and hold operation at the encoder This will give rise to quantization distortion as well as the sampling distortion already mentio
6. hen a pure sinewave is used It will be difficult to make one of these here because our messages have been restricted to rather low frequencies which are outside the range of most TIMS modules But there is provided in the PCM ENCODER a message with a shape slightly more complex than a sinewave It can be selected with the switch SW2 on the encoder circuit board Set the left hand toggle UP and the right hand toggle DOWN See the TIMS Advanced Modules User Manual for more details A message reconstruction LPF is installed in the PCM DECODER module version 2 and above frame synchronization In all of the above work the frame synchronization signal FS has been stolen from the encoder as has been the clock signal This was not necessary The PCM ENCODER has circuitry for doing this automatically It looks for the alternating 0 and 1 pattern embedded as the LSB of each frame It is enabled by use of the FS SELECT front panel toggle switch Currently this is set to EXT FS 1 change the FS SELECT switch on the front panel of the PCM DECODER module from EXT FS to EMBED Notice that frame synchronization is re established after a short time Could you put an upper limit on this time appendix automatic frame synchronization TIMS Lab Sheet The PCM DECODER module has built in circuitry for locating the position of each frame in the serial data stream The circuitry looks for the embedded and alternating 0 and 1
7. ned Read about these phenomena in a Text book experiment the transmitter encoder A suitable source of PCM signal will be generated using a PCM ENCODER module This module was examined in the Lab Sheet entitled PCM encoding 1 PCM ENCODER on the SYNC MESSAGE switch SW2 set left hand toggle DOWN right hand toggle UP This selects a 130 Hz sinusoidal message which will be used later 2 use the 8 333 kHz TTL signal from the MASTER SIGNALS module for the CLK 3 select with the front panel toggle switch the 4 bit LINEAR coding scheme 4 synchronize the oscilloscope to the frame synchronization signal at FS Set the sweep speed to 0 5 ms cm say This should show a few frames on the screen 5 connect CH1 A of the SCOPE SELECTOR to the PCM OUTPUT of the PCM ENCODER 6 we would like to recognise the PCM DATA out signal So choose a large negative DC for the message from the VARIABLE DC module The corresponding code word is 0000 so only the embedded alternating 0 and 1 bits for remote FS in the LSB position should be seen They should be 1920 ms apart Confirm by measurement and calculation 7 vary the DC output and show the appearance of new patterns on CH1 A When finished return the DC to its maximum negative value control fully anti clockwise The PCM signal is now ready for transmission In a later Lab Sheet the PCM signal will be sent via a noisy bandlimited channel For the present i
8. t will be connected directly to a TIMS PCM DECODER module the receiver decoder 1 use the front panel toggle switch to match the transmitter encoding scheme 2 steal the TTL clock signal and connect it to the CLK input 3 initially steal the frame synchronization signal FS from the transmitter by connecting it to the frame synchronization input FS of the receiver and check that the FS SELECT toggle switch is set to EXT FS 4 ensure both channels of the oscilloscope are set to accept DC set their gains to 1 volt em With their inputs grounded set their traces in the centre of their respective halves of the screen Remove the grounds 5 connect CH2 A to the sample and hold output of the PCM DECODER a DC message Now check the overall transmission from transmitter input to decoder output The message is a DC signal 1 connect the PCM DATA output signal from the transmitter to the PCM DATA input of the receiver TIMS Lab Sheet Copyright 2005 Emona Instruments Pty Ltd 2 4 Emona TIMS PCM decoding L 25 rev 1 3 2 slowly vary the DC output from the VARIABLE DC module back and forth over its complete range Observe the behaviour of the two traces The input to the encoder moves continuously The output from the decoder moves in discrete steps These are the 16 amplitude quantizing steps of the PCM ENCODER This is the source of quantizing noise The output can take up only one of 16 predetermined values
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