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 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
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,
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:
150 ohm ¼ watt resistors – Jaycar Electronics cat no RR-0552
100 pico-farad capacitors – cat no RC-5324
1.5 nano-farad capacitors – cat no RM-7015
amount of punched board – cat no HP-9562
47uH suppression chokes – cat no LF-1274
1.5-metre dual male RCA > dual female RCA – cat no WA-7070
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
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
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 1/8th 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.
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.
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
Don’t take anything for granted when chasing noise....