AutoSpeed - Setting Up an In-Car PC, Part 2

Last week in Setting Up an In-Car PC, Part 1 we gave you an overview of the elements required to set up an in-car PC – an inverter, screen, trackball and FM modulator.

And it’s the latter which we’re concentrating on in this story – getting PC sound into your standard car sound system. Taking the FM modulator approach gives you the convenience of having your system’s normal volume control work as it should and saves you the (potentially mammoth) cost of setting a parallel system of amps and speakers dedicated just for the PC system.

FM Modulators

FM modulators tend to have a bad reputation – isn’t their sound really crappy? Well it is – and it isn’t.

Firstly, compared with (say) CDs, FM radios have a more limited frequency response (they normally roll-off at about 15 kHz) and a more limited dynamic range. However, given that - when they’re tested - most people over the age of about 20 can’t hear much above 15 kHz anyway, the frequency response issue isn’t such a big deal. And in a moving car, the dynamic range that you can actually hear is limited by the amount of ambient noise that’s present.

That’s not to say that an FM modulator will blow away CD sound – it won’t – but in reality, the sound quality available through a decent car sound system via an FM modulator is far better than many believe. (Though of course the quality will depend on how good your FM radio is!)

But what is an FM modulator? Basically, it’s a box that takes in a line level stereo signal and transmits it on a FM radio frequency that can be picked up by your normal car radio. Some FM modulators are literally just radio transmitters – they have an internal battery and line-in connections and that’s it – while others work off 12V, have connections for your car radio aerial, and broadcast directly into the aerial input of the radio. These latter designs normally have an on/off switch – turning the FM modulator on switches the aerial connection to the modulator as well as energising its internal electronics. With this type of approach, listening to the PC requires that you switch on the modulator, select the right FM radio frequency (easy with a preset) and then turn up the volume to listen to the PC music.

FM modulators that directly feed their signal into the aerial have two benefits – reception problems don’t exist, and the output power of the modulator can be reduced, so leaving more room in the box for getting the rest of the system right. As a result, direct injection FM modulators are usually superior to their radio-alone based brothers.

The FM modulator that we used is Jaycar Electronics cat no QM-3780, which costs AUD$89.95. It has two controls – frequency, that can be adjusted for either 88.7 or 89.1, and the output level, which can be varied. The frequency should be set so that it doesn’t clash with any existing radio station and the output level adjusted so that the PC sound is at the same level as a normal FM radio station.

Interference...

So wire the FM modulator to power and earth, plug the aerial leads into the back of the head unit and the aerial, and run long RCA line-level signal leads back to the in-car PC. Hey, that’s easy, right?

Yeah, we thought along those lines, too. But there are some big potential problems – well, loud problems, anyway,

A major issue is that PCs put out lots of frequencies that aren’t in the audible band – they’re higher. But like a dog, the FM modulator is capable of hearing those frequencies – and it doesn’t like them.

In fact what results is a heap of audible background noise – it’s not a hiss or a whine, but instead very much like what is known in audio engineering circles as ‘white noise’. White noise has equal audio energy in all frequencies – it’s similar to the noise that you can hear between the stations on old FM receivers (the ones that don’t have auto-muting). What happens is that the high frequencies coming out of the PC interact with the FM modulator to create noise in the background. (In fact, white noise isn’t all that might be heard – see the ‘More Noise?’ breakout box.)

So how do you get rid of this noise? What you need to do is build a filter that rolls off the frequencies above those that you can hear.

Building a Filter

With the help of Silicon Chip magazine’s John Clarke, the following filter was designed and constructed. It‘s easy to build and works well.

These components are needed:

2x 150 ohm ¼ watt resistors – Jaycar Electronics cat no RR-0552
4x 100 pico-farad capacitors – cat no RC-5324
2x 1.5 nano-farad capacitors – cat no RM-7015
Small amount of punched board – cat no HP-9562
2x 47uH suppression chokes – cat no LF-1274
1x 1.5-metre dual male RCA > dual female RCA – cat no WA-7070
1x 64 x 58 x 35mm cast alloy box – cat no HB-5030

Note that the parts are available from a wide variety of electronics suppliers – we’ve listed Jaycar Electronics just for convenience.

The circuit diagram looks like this. None of the components is polarised and the circuit can be physically laid out on the punched board exactly as shown here, making it easy to construct. The inductor and 100 pico-farad capacitors form the radio frequency filter. The 150 ohm resistor isolates the audio output from the 100 pico-farad capacitor and the 4.7 kilo-ohm resistor and 1.5 nano-farad capacitor form the audio filter at 20kHz.

The filter slots in-line between the PC and the FM modulator. To connect the filter up, cut in half the dual male RCA > dual female RCA cable assembly that you’ve bought. Strip the outer insulation cover off each of the four cables (ie two in and two out) to reveal the braid and inner insulated core.

Do the input cables first. The first step is to work out whether they’ll need sockets or plugs on them to connect to the cables coming from the PC. Once you’ve made this selection, join the two outer braids together and connect these to the input ‘common’ of the filter. One inner conductor is then soldered to the ‘input right’ connection and the other to the ‘input left’ connection.

Then do the same for the output cables that connect the filter to the FM modulator.

The filter should be built into a metal box – the metal helps continue the shield around the cable. We used double-sided tape beneath the circuit board to both hold it in place and electrically insulate it from the metal of the box. The box should be electrically connected to the ‘common’ lead.

In some cases you might find that connecting the common wire from the filter to a normal car radio suppression capacitor that is mounted on the chassis (as shown here) further reduces noise. This is basically a suck-it-and-see procedure – if it makes a difference, great. If it doesn’t, leave it off.

When all is working at it should be, use some hot-melt glue to hold the inductors in place – this will stop them placing stress on their supporting leads. The lid can then be put on the box and the filter tucked out of sight under the dash. It’s important that you have the system working when you’re doing these final steps – in some cases, the placement of the box will be significant in giving the least possible noise.

Off the Shelf?

So can you buy a filter off the shelf that does much the same as the one constructed? We bought the Jaycar Electronics Car Stereo Line Isolation Transformer (Cat no AA-3085 at AUD$19.75) and tried it. While not designed as a line-level noise filter, the transformers act as inductors and so it has some effect. The verdict? A lot better than nothing at all but not as good as the home-built filter.

Running the Line Level Cables

As you can now see, noise is a real enemy of a PC-based audio system, so don’t skimp on the quality of the RCA leads that connect the FM modulator to the PC. You don’t need to buy mega-dollar ones, but by the same token, RCA leads picked up from the local discount store may well not be up to the task.

Usually, the sound card in a PC has an 1/8^th inch stereo plug so you’ll need an adaptor cable that has this type of plug at one end and male RCA plugs at the other. In addition, at least one more ‘extension cable’ will probably be needed. Bind any plug-to-plug connections with electrical tape – this will stop inadvertent earths with the chassis and also prevent the plugs pulling apart if the cable is tugged on.

Conclusion

Take care with cable runs, use a quality FM modulator and build and install a signal noise filter and you’ll be able to get good quality PC audio through your normal car system.

Next week: getting the front-mounted LCD up to speed.

More Noise?

Noise in audio systems is a funny thing – it can creep up on you right when you’re not expecting it. All sorts of devices can create noise if they’re in contact – or even close – to your signal leads or the FM modulator. That’s why it is very important that when you’re doing the cable runs for your system, you have the car engine running and the system operating and playing music – yep, even as you’re cable-tieing the wires in place.

Tell you a story. There I was, late at night running the RCA lines back to the boot-mounted PC. Spent ages making the installation neat, binding it with electrical tape and fitting it under the trim panels. Trouble was, when I fired it all up, there was this terrible hissy whine in the speakers.

All of the time.

I couldn’t work it out – this was with the noise filter (see main text) in place and previously all had been really quiet in this exact configuration.

So I started investigating where the noise was coming from. The first discovery was a real doozy – when I moved my mains-powered lead-light fluoro away from the car floor, some of the noise disappeared. Put it back and back came the noise. Moral of the story – use an incandescent (filament) light to show you the way when putting the system together.

But even with the fluoro light gone, there was still heaps more noise than there should have been.

So I dismantled the centre of the dash, something that I’d just spent an hour or so reassembling. Inside were the FM modulator, its power supply and the LCD power supply. The LCD power supply (a device that ensures no battery surges or over-voltage spikes get to the LCD) is housed in an enclosure (pictured) about the size of a box of matches. I initially couldn’t believe it, but I found that if this box was closer than 50mm to the FM modulator, up came a shrieking, hissy noise in the speakers...

But it gets even worse. Switch off the LCD and the noise dropped even further – the power supply was not only radiating noise, but was also injecting it back into the car’s wiring harness. We’ll cover how this problem was overcome in next week’s article.

Don’t take anything for granted when chasing noise....

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