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My Review of the Broadcast Warehouse DIGILOG MPX Kit


13th Oct '99
My review of the Broadcast Warehouse DIGILOG Stereo Multiplexor kit
( To be read in conjunction with my "Stereo for Dummies" page )

The bottom line first :
Once again, Broadcast Warehouse have raised the performance expectations of kit builders by releasing a professional quality kit. The DIGILOG uses advanced but well-known methods to increase fidelity and channel separation during the MPX generation process, making it the most sophisticated MPX kit in the market. Documentation is much more detailed than seen with other kit manufacturers. Wholeheartedly recommended.

Preamble :
By default, Scott Masters, Chief Designer at Broadcast Warehouse automatically sent me a free DIGILOG Stereo MPX kit for review when it was released around end February 1999. I must confess that at that time, I did not clearly understand the subtler nuances of generating stereo signals, preferring to look at Exciters and RF Amps instead. After playing around with some other MPX kits and doing a lot of reading up on Stereo since Feb, here's my review finally...

Why call it the DIGILOG ? :
The basic design goals of a TDM Switching based Stereo Multiplexor are -
A) Have a stable, exact 19 KHz Sine wave Pilot Tone.
B) Internally generate a 38 KHz signal which is in phase with the 19 KHz Pilot. This is needed for the Multiplexor.
C) Time Division Multiplex the Left and Right Audio channels generating the least amount of harmonics.
D) Have a MPX Low pass filter that causes the least phase shift in the MPX signal, while still effectively removing unwanted harmonics in the MPX signal.

Analog solutions do generate fewer harmonics and have a cleaner signal. On the other hand, it is expensive and cumbersome to implement a rock-steady analog 19 KHz pilot sine wave and also the 38 KHz signal in exact phase with it. Small tolerances in parts and the inevitable deterioration with time make the two signals shift in phase and frequency, requiring expensive test equipment to set up and re-tune.

Traditional fully digital solutions to the above design challenges do excel in creating very steady 19 KHz square waves to use as the Pilot Tone and 38 KHz square waves for the TDM. However a digital solution outputs an MPX signal that looks like a rectangle wave. Square waves and Rectangle waves are, by definition, rich in odd harmonics. Having extra harmonic content muddies the MPX signal and makes the transmitter waste its output power. Trying to filter an MPX signal with too much harmonic content necessitates a sharp filter that causes tremendous phase shifts in the signal. This leads to loss of channel separation at the FM Receiver.

Clearly, the best solution lies somewhere in between the DIGItal and the anaLOG domains. Obviously Digital electronics should be chosen for creating steady 19 KHz and 38 KHz signals and the basic MPX signal. However, one could use techniques similar to Digital to Analog Converters to convert the 19 KHz square wave signals to a stepped "digital sine wave". Similarly weighted oversampling techniques can be used to convert the basic MPX signal to a smoother signal.

The above techniques generate signals closer in shape to the analog signals we really want. That means lower extraneous harmonics to begin with. One can now use a gentler MPX filter which creates lower phase shifts. This leads to better channel separation.

The above techniques are not new. Professional Stereo Coders have been using exactly the same technique for the last 10 years, using 4 or 8 bits for generation of the Pilot and the MPX signal. What Broadcast Warehouse have done is effectively bridge the vast gap between the really low end kits out there and the expensive Professional gear, using appropriate engineering compromises to greatly improve performance while still reining in costs. Kit builders now have access to a semi-pro MPX kit still in the same price ball-park as other manufacturer's low end kits.

Clever Circuitry :
Scott decided to use 4 bits ( 16 states) to generate the 19 KHz digital sine wave pilot and 2 bits ( 4 states ) for the MPX signal. Instead of using a lot of discrete digital chips to generate the Pilot, the Chopper signal and all the timing signals for the TDM, Scott put in all that functionality into a 16C52 PIC ( similar to the one used in the 1W PLL Exciter) for a software implementation. As the PIC has 8 ports, it conveniently can use 4 ports for the Pilot DAC and 4 ports to handle the TDM.

It appears that using PICs instead of lots of hardware is one of the hallmarks of Broadcast Warehouse products. Programming PICs need a different skill set than just bringing together chips for a hardware implementation. The resultant final products are cheaper, easier to modify and customize and use less PCB real estate.

The actual TDM is based on a MAXIM DG411 high speed switch controlled by the PIC. These switches can work upto about 4 MHz switching cycles but the DIGILOG has them working only at 304 KHz ( 38 KHz * 4 states* 2 times per cycle), so they are fast enough for the application. Slower switches would generate more intermodulation distortion products due to larger ON and OFF times.

But I have spent my time describing what really makes the DIGILOG tick. Lets begin from the beginning. Input Audio signals are sent to an active pre-emphasis stage with switchable pre-emphasis values. That is followed by a passive 15 KHz LPF brickwall filter. The next stage is a clipper.Outputs from the Left and Right channel clippers are fed to the DG411 switch that TDMs the signal. The Pilot tone is added to the output of the TDM using low noise Opamps. This MPX signal is then fed to a passive MPX filter. A final buffer stage presents a fixed impedance to the MPX filter output, while itself presenting a low impedance as the final output of the DIGILOG.

The DIGILOG is obviously Scott's Labor of Love. Lots of nice little touches and attention to details make it a very versatile kit.

- Power supply between 8-15 Volts without modifications
- Built in voltage regulators
- Expansion port allows connection of other kits to DIGILOG's +12/0/-12V power supply
- Balanced or Unbalanced Audio inputs
- Switcheable pre-emphasis ( None / 50us / 75us)
- Switchable Clipper ( None, All, high frequency only)
- Pilot out port to connect to external systems that need a time signal
- Stereo/Mono Jumper

The PCB and components are of high quality. If you decide to obtain the unbuilt kit option, keep aside about 6-8 hours to solder in the components, as there are quite a few of them, and quite close-by too.

Documentation :
The 10 page documentation is very detailed. While it does not go into the technical nitty gritties of the design, it very clearly explains the necessity and the usage of the various blocks that make up the whole. The setup procedure for Stereo Coders is always a bit more cumbersome than, say, a no-tune exciter. Scott has explained the various procedures and options very completely, allowing the user to make informed decisions on setting up his kit.

The Documentation includes a complete circuit diagram with all part values, making it the first such B/W kit. I take that as a small victory for all of us in my personal crusade to get all kit manufacturers to release circuit diagrams. The final goal would be to have circuit diagrams available at manufacturer's website so that potential purchasers could judge manufacturer's claims for themselves before purchase.

All in all :
With the DIGILOG, Broadcast Warehouse once again remind the market that they are primarily a professional RF company that also makes kits, rather than the other way around. You want to do stereo correctly ? Go buy a DIGILOG !!


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HOME PAGE

My review of the Broadcast Warehouse 1W PLL Exciter with LCD 08 Aug 1998
Alt'F reviews the Broadcast Warehouse DIGILOG Stereo Multiplexer Kit

Scott's further comments re: encoding / decoding Stereo
USENET messages about this Page
My comments on the BH141x FM Tranbsmitter chips by ROHM 26 Aug 2000

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