Remembering NICAM Part 2: Recreating a PAL-I signal, and receiving it with something fancy

In my previous post we took a look at some of the best equipment available for NICAM transmission. Now we’re going to take a look at the receiving end.

NICAM by its very nature delivers us to the world of the audiophile, where the prospect of a 1/1000th improvement in audio quality demands a £1000 device to capture it. Today I’ve got just such an item on the bench: The Arcam Delta 150:

Before we get into the test setup, let’s whip the lid off this thing and take a look:

The Delta 150 is essentially a terrestrial television receiver, including NICAM audio with some extra signal processing magic thrown in. It was designed in the UK and is hardwired for the PAL-I standard – limiting its use to the UK, Ireland, Hong Kong and South Africa. I doubt many migrated from these shores.

SAA7220P TDA1540
Arcam’s design, crudely annotated in MS paint.
Arcam Delta 150 block diagram. Oopsy daisies. There’s an error in it which I’ve corrected. It could also have been designed the way the block diagram originally suggested but the one I have wasn’t.

How it works

For the most part the design of this unit is quite typical of any terrestrial receiver at the time. The SAW IF filter after the tuner has two outputs, one covering the IF frequency of the vision carrier. This is fed into the TDA8341 whose job is to recover the composite video signal. We don’t really care about this part. Let’s move on.

The other output of the SAW IF filter heads over to the TDA2546A which contains the FM mono receiver. This chip also outputs a second IF from which the 6.552 MHz (PAL-I) NICAM carrier is selected. This signal then passes through the 4 pole Toko filter (specifically designed for NICAM) then to the TA8662N DQPSK demodulator.

The TA8662N outputs the baseband NICAM data and recovered clock from the transmitter (the very same signals outputted from the back of the PM5687’s “NICAM OUT” BNC connectors) then feeds it to the CF70123C, which is the NICAM codec. The signal should then head onto the TDA1543 DAC, but it doesn’t.

This unit has an extra chip which sits in between the codec and the DAC: An SAA7220 from Philips. The special sauce of this unit. Its datasheet describes it as “Digital filter for compact disc digital audio system”.

This filter is intended to reduce (not necessarily eliminate) the anti-aliasing filter at the output of the DAC to satisfy the Nyquist–Shannon sampling theorem. Instead of having a large and complicated analogue filter network rammed with obscure and expensive adjustable components, instead things are mostly done in the digital domain with a simple chip the manufacturer can whack on the board without requiring any manual adjustments on the production line. A simpler filter arrangement after the DAC may remain.

Philips’ second (and third) generation NICAM decoders (the SAA7282 & SAA7283) had a built-in DAC adopting a 1-bit architecture, rendering this approach and any of the original benefits of the Delta 150 redundant.

After that we’re of course at the TDA1543 DAC. A popular but austere DAC from 1989 often found in older CD players. It is interesting that we don’t see more money thrown at this component however given that the source ADC is only 14-bit this might have been hard to justify.

To get a sense of how typical the Delta 150 is, we needn’t look any further than Mark Hennessy’s home-made receiver. It uses the same components and design. There is little, if anything to set them apart. Mark even one-upped Arcam packing all of this into an elegant 1RU enclosure.

Getting it working (Intercarrier method)

So how do I bring the Arcam back to life, using the unfamiliar connections on the back of my broadcast NICAM equipment?

I reached out to Nathan Dane from Ireland who’s also recently been looking at this and was informed of a “hack” to make it work quick and easy:

PM5686A connected to the Arcam via a Labgear MOD111 modulator.

Above, the Intercarrier output from the PM5686A is simply connected to the composite video input of a cheap CCTV modulator, which in turn is feeding the Arcam. This worked first go and required no special expertise or equipment.

Feeding what is effectively an audio signal into a video input is a little counter-intuitive. Let’s bust out the spectrum analyser and see what’s going on here:

Intercarrier output of the PM5686A.

Looking at the PM5686A’s intercarrier output we can see a modulated NICAM carrier at the absolute frequency of 6.552 MHz. The exact spacing from the vision carrier in a PAL-I system.

The CCTV modulators output. The peak on the left is the empty video carrier, the tall peak on the right is the empty FM mono carrier

Now when we look at the CCTV modulator’s output with an empty video carrier at 471.25 MHz. We can also see that familiar NICAM carrier again to the right, after the empty vision/FM mono audio carriers. All the modulator does to the video signal is shift the frequency up by the carrier frequency, putting the NICAM carrier at 477.802 MHz. Exactly where it needs to be.

The beauty of this method is that the Arcam won’t tune to just a NICAM signal by its self. It requires the accompanying PAL-I transmission, which has been provided.

The drawback however is that the composite video input is occupied by the NICAM connection. To have both video and NICAM on this input would require some additional circuitry which I won’t be looking into at this time.

I doubt this method would work on a Broadcast grade, or even a higher quality vision modulator as such equipment is likely to filter out signals above the maximum allowed bandwidth for composite video.

RF Method

In a large scale setup, the carriers are more likely to be combined on-channel as RF signals. To make this work I needed to put together a makeshift upconverter. Throwing it all together – I now have this stack up:

The NICAM signal is now fed into the antenna input of the MOD111 having just been upconverted by the mixer on top of the PM5686A.

The mixer is driven by my ESG signal generator. As overkill as this appears its ultra fine frequency granularity allows me to exactly match the error of the MOD111.

The signal generator is set to 510.1525 MHz – which is 471.25 MHz (UHF channel 21) + 32.348 MHz (PM5686A IF frequency) + 6.552 MHz (PAL-I NICAM offset) + 2.5 KHz (oscillator error in the MOD111).

And we’re in business. The “NICAM” LED on the Arcam receiver is illuminated once again. Did I go to all of this trouble just for the LED? Of course not. I’ll be listening to some music through this setup for the next few days.

After much fussing around I can now present a full PAL-I spectrum including NICAM. Something that hasn’t been seen for a long time 😉

The four peaks shown here are (from left to right): Luma (Vision), Chroma (Vision), FM mono audio and NICAM.

Up next

You thought your Arcam Delta 150 was the be-it-all-end-all receiver? Think again.

Posted in Broadcast tech, NICAM, Vintage audio

7 thoughts on “Remembering NICAM Part 2: Recreating a PAL-I signal, and receiving it with something fancy

  1. There is a faint hiss in the output of my Delta 150, even when decoding NICAM. It’s only really noticeable during quiet or silent moments of audio. I’m curious if yours is the same or if I just got a bad one? All my TVs with built-in NICAM decoders don’t have it.

    1. You are somehow still able to use a Delta 150? or was this from memory? I can check mine at some point but haven’t got the test-setup assembled at the present time. Will report back when I next look at it.

      1. Still able, I wrote a little program called hacktv that can generate the RF signals for various analogue video standards and transmit them with an SDR device (usually a hackrf). It has basic NICAM support (stereo only, no mono or dual language).

        1. Just a little program? Yes I have come across that. I was curious as to how I might test the data output on the PM5688. Need a bespoke coder for that. Perhaps I could adapt your project.

          I have to say it is quite a contrast to see 10 kilograms of sweat and bare metal from 1985 distilled down to 475 lines of code.

        2. By the way there is a tiny bit of hiss on the Delta 150, but it’s way better than a NICAM decoder card from a TV, which I also had, based on the same DAC.

          In comparison The PM5688 has zero hiss, but the difference is tiny. If I disable the carrier at the source (i.e. stop the decoder), there is no change in the hiss.

          1. Well it used to be a small program 🙂 Not as nice as using actual broadcast gear though. If I could find them and had the space, I totally would too.

            Thanks for checking the Arcam – from the description I’d say yours has less noise than mine, so I might open this one up and see if any of the capacitors are looking tired.

          2. I had to turn the volume on the amplifier up very high to hear any noise suffice to say it is minimal.

            You may find it’s noise from the digital circuitry being coupled into the analogue stage. Capacitors could be the cause.

Leave a Reply

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