Remembering NICAM Part 1: Broadcast equipment teardown

NICAM Digital Audio first went on air in 1986 and is one of the, if not the first digital broadcast received by households in much of Europe and Asia/Oceania (excl. Australia). It is well remembered by both AV enthusiasts and audiophiles for providing near CD quality audio alongside analogue television broadcasts. For my teenage self, watching the music video charts with the stereo attached to a NICAM receiver was always a highlight of the week.

NICAM: A lot more than just a random marketing slogan slapped on TVs and VCRs. Definitely nothing to do with cameras.

It was originally developed by the BBC for FM radio backhaul and likely still used for that to-date. There are many pages around the internet discussing its theory and history. Myself, I’m keen to get my hands on some of the kit, look at it and see if I can make it work.

For the most part, NICAM broadcasts ceased around a decade ago. With this technology obsolete, the original broadcast equipment is now scrap, obtainable within a curiosity budget. Finally people like myself can actually get a look at it.

NICAM vs Zweikanalton vs MTS

Before I start taking things apart, it’s worth covering this briefly. Zweikanalton was another stereo TV sound standard which competed with NICAM in the PAL and SECAM world. Its biggest adopters were Germany, Australia and the Netherlands. They are functionally similar however Zweikanalton is entirely analogue. It had the advantage that receiving hardware was little more than an FM stereo radio, and was easy and inexpensive to manufacture. Transmitting equipment was also backwards compatible with earlier FM mono receivers.

NICAM on the other hand required a second disparate transmission path and a complicated and expensive receiver arrangement not included in older or more affordable receiving equipment, for which the traditional FM mono carrier had to continue to be broadcast. This meant that many consumers missed out on stereo sound due to budget constraint, ignorance or indifference.

For example, TV equipment intended for the Australian market (Zweikanalton) was commonly found in New Zealand (a NICAM country) labelled “HiFi Stereo”, but only delivering poor quality mono sound because the in-built demodulator was only able to receive the legacy FM mono carrier. This frustration was likely shared by European consumers from NICAM countries neighbouring Germany and Netherlands as economic forces shunted cheaper Zweikanalton equipment over borders.

MTS Is another analogue stereo system like Zweikanalton used exclusively with NTSC and PAL-M (effectively NTSC) broadcasts.

Philips PM5588 Zweikanalton (A2 Stereo) modulator in-situ. An evil bit of kit which had no place in the lands of those who care about their sound quality.

One of the biggest drawbacks of Zweikanalton was that the audio had to somehow be transported to the site of transmission as an analogue signal, recalling the very problems the BBC were trying to solve with NICAM.

NICAM Broadcast Equipment

There are typically at least two pieces of equipment involved in NICAM broadcast, in different physical locations linked over a leased line or microwave link. The top unit at the transmitter, the bottom unit at the studio

On this page I’ll be looking at two pieces of equipment from Philips introduced at the IBC-88 trade show. The samples I have are dated from the early 1990s:

  • PM5687: Studio component: Digitises analogue audio and encodes it to NICAM. Optionally it can also modulate a ready-to-broadcast carrier if fitted with Unit 6 (details below)
  • PM5686A: Transmitter component: Modulates pre-encoded input from PM5685 or PM5687
  • PM5688: Test NICAM receiver (separate post)

The BBC appear to have built their own hardware. There is a picture of it here. I doubt many other broadcasters did.

There are several related pieces of equipment I do not have, examples of which were for sale on eBay at the time of writing:

  • PM5685: Cut down version of PM5687. Digitises and encodes only (Unit 6 not fitted)
  • PM5680: Accompanying vision / analogue audio modulator

A detailed look at the PM5687

The PM5687 speaks the design language of another era, consisting almost entirely of off-the-shelf through-hole components. Everything is done the hard way. It does however pack everything into a single 2RU enclosure. The components within date it around 1992 – for most adopting countries the beginning of the NICAM era

Later on, even more refined solutions appeared like the Factum Electronik NC200A which uses surface mount components, custom ICs and offers two channels in a single 1RU enclosure.

I do not have the manual for this item. I couldn’t find it online either. I suspect it only existed on paper, and it wasn’t included in the sale. Thus a lot of what I’ve stated here is from experimentation or guesswork.

Internal components

Despite being 30 years old, it doesn’t need recapping. I checked.

Looking in the top with the cover removed we can immediately see some classic money-is-no-object engineering. The unit has a very complicated structure of machined/extruded/cast aluminum parts held together with more than a hundred screws, weighing in at over 10 kilograms.

Undoing the two screws on the front panel I discover that it folds down, revealing the delightful mechanical construction allowing the internal modules to be removed by simply removing the screws and pulling the eject lever.

There are three modules each connected to the backplane with DIN41612 60 Regular / 4 Coax connectors and an empty slot for a mystery fourth.

Unit 3 – Audio Input

This module’s job is to take the analogue audio signals from the rear inputs and digitise them, ready for the codec which we’ll be looking at shortly. We’re immediately confronted by an enormous filter network on a mezzanine PCB. I haven’t studied it in detail (yet) but I think it is a Chebyshev or Butterworth filter using a swag of adjustable inductors and ultra-stable polystyrene capacitors. It would have to have been manually tuned on the production line.

It’s job is to deliver every precious hertz of audio spectrum to the ADC appropriately equalised, blocking anything it cannot digitise (i.e. >16 KHz).

I attached my headphones to the two test points at the output of this board to listen to what is delivered to the ADC. It seems to prefer higher frequencies, attenuating those at the lower end, even with the pre-emphasis switched off. This effect is somehow cancelled out by the time the audio reaches the listening end.

The most prominent chip on this board is the TDA1534 ADC. I was hoping to find a study of its performance but unfortunately ADCs rarely attract the intense scrutiny of the audiophile community. Regardless, this is a very, very special ADC. Not necessarily by specification, but by its job.

Removing the filter mezzanine we can now see all of the analogue goodies underneath. There are few components which aren’t made by Philips. There are some here made by Signetics however this was owned by Philips at the time this was made. Even the capacitors are all Philips. I never knew they made these too.

This was just me having a quick squiz. Feel free to correct me if I’ve got anything wrong.

Here they are annotated.

Audio board rear
Audio filter mezzanine rear

Unit 5 – Baseband Coder

Additionally there are two EPROMs, three SRAMS, a FIFO and a huge tangle of 74xx logic.

This board was a surprise for me. It’s effectively the NICAM codec (“coder” in the language of the day), implemented in software. Philips drafted in what at the time would have been a grunty 68K processor for the task. Gaining a detailed understanding of how this module works would be quite an undertaking. However it works, it has to do things darned quickly else there would be issues with lipsync.

Codec PCB rear

Unit 6 – QPSK Modulator (Optional)

This board’s job is to turn the digital bitstream generated by the codec into an RF signal (a modulated DQPSK carrier) that can be broadcast. It is unlikely to be required at a studio location where this unit would normally be situated, thus, Philips also offered a version of this equipment without this module fitted with the model number PM5685.

It is this board which decides whether or not a carrier for the CCIR System B/G/D/K (PAL/SECAM) or System I (PAL I) standard is produced. This one is setup for B/G. It probably could be adjusted however I’d rather not touch it as it has likely been tuned to absolute perfection by experienced Broadcast engineers. I have neither the skill set or documentation to do so myself.

We can get a sense if how it operates from page 24 of the NICAM specification. It’s quite interesting. It appears that the I/Q signals are being generated by DACs which in turn are fed by tables loaded into the two Bipolar PROMs whose address lines are driven by the maze of nearby 74 series logic.

There is a mystery Philips chip which appears four times on the board: OQ0702P – hampering my attempts to understand it. It’s probably performing harmonic signal mixing.

DQPSK modulator board rear

Frequency reference (master oscillator)

The PM5687 did not disappoint here either. Packing a 10 MHz TCXO from Philips @ 5 ppm. Not enough to satisfy meteorology buffs but more than good enough for broadcast.

There’s a couple of fractional dividers to provide the 5.842 MHz clock (for the NICAM encoder) as well as the 200 KHz clock which appears to head to the DQPSK modulator. Performing divide is the ‘HC4059 counter and the trusty ‘HC4046 PLL for multiply. Notice that the passives normally required for the ‘HC4046’s VCO are missing. Philips apparently weren’t satisfied with the in-built VCO so disabled it and rolled their own based on an exotic CA3130 CMOS op-amp. From a quick bit of research this was a technique to build an ultra-stable VCO back in the old days.

The internal 10 MHz oscillator can be disabled, making the 10 MHz output at the rear an input, if an even more precise 10 MHz reference is available.

Oscillator rear

Front panel

WIth VU meter attached. This assembly is all digital. The analogue part of the VU meter is the PCF8591 on the analogue board.

Removing the VU meter we can see the whole unit is driven by a Philips branded MCS-51 microcontroller bolstered with external SRAM and EPROM clocked at a very generous 12 MHz. Given that it’s got to drive the VU meter and deal with incoming SCPI commands this was probably needed.

GPIB Interface

And it’s a Philips branded MCS-48 on here. This one appears to be Philips version of the Intel 8021. A mask ROM version with 28 pins. There’s no hope of dumping the code from this one but it appears that isn’t necessary. The MCS-48 on the front panel appears to be the one interpreting the SCPI commands as I can see them all in the ROM dump.

GPIB board rear

Front panel

Controls on the front panel are as follows:

INPUT

Switches between analogue inputs and pre-encoded digital inputs (NICAM IN).

MODE SOUND 2

Toggles between A/B as input, A/C as input or A only. Data setting purpose unknown.

“Sound 2” refers to NICAM. “Sound 1” is the FM mono audio not handled by this unit.

SOUND FLAG

At a guess I think this probably toggles the “The reserve sound switching flag” detailed in section 4.2.2.3 of the NICAM specification. This tells receiver that the FM mono carrier is carrying different audio entirely to the NICAM carrier. An obscure, rarely activated scenario.

AUDIO

Switches between analogue audio input or test tone generated in software by codec.

PRE EMPH

Explained here.

SOUND CARRIER 2

Enables/disables internal DQPSK modulator. Allows the broadcaster to disable NICAM and force receivers to fall back to FM mono audio.

Rear panel

AUDIO A/B IN

Left and right audio inputs in stereo single language mode.

AUDIO C IN

This input is intended for bilingual broadcasts and is the input for the secondary language sound track. In bilingual mode the transmitted B channel is swapped over to this input, the A channel then becomes the mono primary language sound track.

AUDIO OUT

For connection to the legacy FM mono modulator. It is either the A+B channels combined (single language mode) or the A channel only when the B channel is being used for a second language.

In some cases the FM mono modulator wouldn’t be in the same location as this unit. It would be interesting to know how this was dealt with.

IF OUT (Intermediate Frequency)

Broadband NICAM output modulated onto a 33.05 MHz carrier. This is for a 38.9 MHz IF system (where the vision carrier is at 38.9 MHz) intended to be directly combined with vision/audio IF carriers before up-conversion (or after, depending on the scale of the transmitter). Spectrum is inverted compared to intercarrier output.

IF IN

Connected to the same passive combiner the IF output connector is attached to. This could, for example, be connected to the IF output of the analogue audio/vision modulator.

NICAM OUT/CLOCK OUT

Baseband output from the codec. Used where the carrier is modulated in a different location to where the audio signal is digitised / encoded.

NICAM IN/CLOCK IN

Baseband input to the internal modulator. Allows the unit to be used as a modulator only in the case where the audio signal is digitised / encoded elsewhere. The internal modulator is switched between the internal codec and the connector on the front panel.

INTER C. OUT (intercarrier)

Outputs the original 5.85 MHz NICAM carrier (for B/G models like this one) for scenarios where video/FM mono/NICAM are all modulated/upconverted together. For the PAL-I model the intercarrier frequency is 6.552 MHz.

REF IN/OUT

Internal 10 MHz reference output, or in the case where it is configured as an input, it would likely be connected to an atomic clock.

IEEE 488

GPIB remote control port. For example, the studio may have some sequencing system that needs to change the configuration of this unit when a bilingual broadcast is on air.

Testing the PM5687

The first thing I checked when powering on was the IF output:

This image has an empty alt attribute; its file name is power.gif

As expected, there is a carrier output 700 KHz wide, at 33.05 MHz

Constellation, BER, Eye and demodulated spectrum

The output demodulates perfectly.

There was an entertaining discovery testing the IF power, It’s -20 dBm. Exactly.

That’s about all the testing I can perform with the instrumentation I have to hand.

A detailed look at the PM5686A

This piece of equipment would have normally been situated at a transmission site hanging off an E1 leased line with a PM5687 or PM5685 on the other end back at the studios. It is an entirely digital device which does not handle the audio in analogue form.

After removing about three and a half thousand screws I finally got the top cover off:

Like the PM5687, it’s delightfully primitive

The mechanical construction did not disappoint here either. To remove each module was simply a case of removing the one screw in the centre of the PCB, sliding that PCB forward a half an inch then effortlessly folding it out. They even attached a pair of machined knobs to each card to assist with the disconnection / connection / raising / lowering of the cards.

The internals were almost as expected. It’s centered around the same DQPSK modulator module found in the PM5687. There are some minor differences – the main being that this one is setup for PAL-I.

Oscillator / Clock recovery module

To the left we a jungle of 74 series logic which performs clock recovery from the incoming NICAM signal as the unit does not accept a separate clock input. This gives the customer maximum flexibility as to what carries the digital NICAM signal as only a single connection is required.

Interestingly the oscillator is 2 ppm. Better than the 5 ppm seen on the PM5687. This makes sense as the stability of the broadcasted carrier matters more than the codec clock.

Oscillator / clock recovery board rear

DQPSK modulator

Very similar to the one found in the PM5687. Jumpers of that right hand side counter are set differently. This is possibly how the carrier clock is set, but unlikely to be the only change to achieve this.

IF Amplifier

The presence of this further bolsters the case of this being from transmission site use. It’s got a big scary looking RF power transistor on the underside.

Oh err, are those surface mount resistors on there? Did you not check the calendar? It’s 1992!

Measuring the IF

The IF frequency of this unit is 32.348 MHz. 702 KHz lower than the PAL B/G model as we would expect. Bear in mind that the IF output is spectrally inverted hence the lower frequency.

IF output with 10 MHz span. This is with the IF level knob turned down to the minimum, at which point that assembly becomes an attenuator. I’m a bit suspicious.

I was interested to find out what kind of output power we’re seeing from that IF given there’s an amplifier in this unit, and the large the size of that transistor on it. I dug out my power attenuator, cranked the dial on the front to the max and prepared to be vaporised:

That’s it? 7 dBm? So little power that I can attach my 8481A with no attenuator. I was expecting at least a couple of watts!

Up next

In part 2 I’ll be looking at receiving equipment, and of course getting it working.

Leave a Reply

Your email address will not be published. Required fields are marked *