Step By Step 13 - Getting Sound Right
Originally published in Australian Personal Computer magazine, November 1998 Last modified 03-Dec-2011.
Pretty much every PC these days has high quality 16 bit audio output. So why is computer sound, generally, lousy?
There are three basic factors that determine the quality of what you can hear from your home hi-fi or PC: Source quality, amplification noise and distortion, and speaker quality.
What you do hear at the end of all this is limited by the quality of your hearing. In youngsters, that covers a frequency range of roughly 20 Hertz (Hz, or cycles per second) to 16 kilohertz (kHz), with high frequency response dropping off steadily as you age. Each musical octave represents a doubling of frequency.
The source
Audio source quality for computers varies. Most PCs have at least a Sound Blaster 16 or equivalent sound card installed, which means they can play digitised sound with 16 bits per sample at very high sample rates. The more bits per sample there are, the more finely the digitised sound can follow the original analogue waveform, with 16 bits generally accepted to be enough. And the more samples per second there are, the higher the maximum reproducible frequency, which is half of the sample rate. CD quality is 16 bit stereo at a 44.1 kilohertz sample rate, giving a maximum reproducible frequency of 22kHz, well above the highest threshold of hearing.
This doesn't, however, mean your sound card will be playing audio of this quality - a lot of digitised computer sound, particularly in older games, is considerably less than CD quality. This is because lower quality takes up less memory, disk space and processor power. An eight bit 22kHz explosion sounds very nearly as good as a 16 bit 44kHz one, especially when there are umpteen other explosions going on at the same time.
MIDI music sound quality can vary enormously between sound cards. The old-fashioned FM synthesis used by the abovementioned bog standard SB16 and many other cards sounds tinny and unrealistic. Wavetable synthesis, the standard on classier sound cards, uses digitised samples of real instruments for its sounds, and sounds as good as the samples allow it to - a wavetable card with plenty of memory for its samples can sound very realistic. Sound cards which use "physical modelling" use much gruntier on-board hardware to actually imitate the sonic behaviour of a real instrument, and can sound the best of all, not to mention be reprogrammed to produce an amazing variety of sounds.
Any PC with a correctly connected CD-ROM drive can play CD audio. The quality of the CD audio has little to do with the quality of the sound card, because it's played by the CD-ROM drive, not the card. The card just channels (and amplifies, unless you're using line out) the analogue signal from the CD-ROM. More recently, CD-ROM drives that can deliver digital audio directly have become common, so it's possible for a computer to play audio CDs without using a simple analogue connection from the CD-ROM drive. The same principle still applies, though; the sound card isn't making the sound, it's just delivering it.
Either way, CD audio played through the computer is generally just as good, for all but hi-fi purist golden eared types, as the same audio played through a dedicated audio CD player. Computer CD-ROM drives in audio mode often deal poorly with scratched discs, and the sound card amplifier (if you're using it) can add noise and distortion, but by and large CD audio sounds great, even on old systems.
Inadequate amps
Amplification noise and distortion can be high in PC applications. If you have powered speakers, they'll have their own amplifiers built in and these are often very cheap and nasty. If your speakers are unpowered, you'll need a sound card with an onboard amplifier (any sound card with a "speaker" as opposed to "line" level output has an amplifier), and onboard amps are usually lousy, too. But this is less of a problem than you'd think - given the raucous nature of most computer sound, quite high amplifier distortion figures are not noticeable.
Small cheap amplifiers also have lousy signal to noise figures - since computers are very electromagnetically noisy items, cheap sound card and speaker amps often pick up interference from the computer. This expresses itself as hiss and other semi-random sounds which are audible during quiet moments. Again, this is no big deal for most users, as most computer sound is well above this "noise floor".
Fix the speakers!
By far the most important limiting factor in most computer audio applications - and in pretty much all hi-fi applications, too - is the quality of the speakers. The speaker in the average TV puts most PC speakers to shame.
A lot of terminology is shared between computer audio hardware and hi-fi gear - especially the lower end, appliance store kind of hi-fi, which purists tend to refer to as mid- or lo-fi.
A selection of speaker "drivers", the actual sound-producing units. Clockwise from top left, there's a pair of four inch units as found in cheap car stereos and larger budget-priced computer speakers; a couple of four inch magnetically shielded drivers as used in computer and television speakers; a couple of nasty little three inch drivers as found in any number of computer speakers, and a pair of vastly superior carbon fibre four inch units with far higher power handling and, as you can see, much bigger magnets. Better computer speakers have drivers of this calibre, but you have to spend more to get them. These last drivers retail for around $AU40 each, and handle 15 watts RMS; the others cost, at most, five dollars each retail, and handle two to three watts.
First, there's power handling, which is how much power you can put into a speaker before it blows up. A speaker with higher power handling is not necessarily louder. If all other things are equal, the higher power speaker can be wound up louder, but if the lower power speaker is more efficient, you'll get more sound out of it for less power input. The statistic to look for is a "watts RMS", which is the Root Mean Square or, in English, "real" power rating. Peak or Peak Music Power Output (PMPO) ratings are all lies. The standard sound card amplifier puts out about two watts per channel RMS.
Efficiency is one of the things you should look for ahead of power handling in a computer speaker. It's expressed in decibels (dB), generally as the output sound pressure measured at one metre for one watt of input power. The decibel is a logarithmic measure, so a three dB difference is a twofold power difference - to get 3dB louder you need twice as much power. High-end computer speakers might actually have efficiency listed on the spec sheet; cheaper gear will have no numbers or some rather optimistic ones. For reverence, a symphony orchestra at full fortissimo, heard from a good seat, is about 90dB. 90dB from one watt at one metre is on the high side for hi-fi speaker efficiency, but from two watts at closer distances, as is usually the case in computer applications, 90dB is thoroughly attainable. For every doubling of distance you need another 6dB to maintain the same sound level.
By and large, the larger the "drivers" in the speakers - the cone-shaped things that make the noise - the more efficient the speaker will be. Yes, that does mean that a hi-fi speaker with a 12 inch bass driver will be a lot louder for the same input than a PC speaker with a three inch driver.
With a reasonably efficient speaker, a weedy little two watt sound card amplifier is more than adequate for room-filling sound. As a matter of fact, most hi-fi listening is done at power levels well below five watts per channel.
The other important speaker statistic is frequency response, which is another thing about which cheap speaker specifications usually lie. Most adults can't hear above 14kHz, but that's not where the lies are. Cheap speakers tell fibs about their bass response.
No matter what a cheap three inch driver's specifications say it does, it has no useful response below 120Hz, and probably bog-all below 200Hz. Yes, it will make a noise if fed a 50Hz tone. But it's so inefficient at that low frequency that it'll barely be audible, even at full power. If you want real response below 100Hz from small speakers (most bass drums are about 60Hz, and engine rumbles and explosions can have much lower components), you have to augment them with a subwoofer of some kind.
Or just use bigger speakers.
If you've got a spare pair of speakers sitting around - hi-fi, boom box, whatever - they're certain to sound as good as anything you spend $50 for at a computer store, and probably a great deal better. If your sound card has an amplifier, all you'll need is a cable to connect the sound card to the speakers. Most speakers have terminals which accept bare wire. Knocking up an adaptor cable from the standard sound card 1/8th inch stereo headphone-type jack to bare wires is a trivial job for anyone at all handy with a soldering iron - here's how to identify the terminals on the plug:
Identifying the terminals on the
standard 1/8th inch stereo plug.
If you've any talent for woodwork, you can easily make your own speakers, with cheap drivers, terminals and other accessories from an electronics store and a few dollars worth of wood.
Headphones are another option. If you don't mind the inconvenience, money spent on headphones will give you much better sound than the same amount spent on speakers.
The coming thing
USB (Universal Serial Bus) speakers plug into the USB connectors on current computers (Win98 PCs, current Macs...), and accept digital audio data from the computer, which they convert to sound with built-in digital to analogue converters. This means you need no sound card at all as long as you only want audio output, not input, and it also makes possible fancy multispeaker setups, with complex positional audio handled by the CPU or a dedicated audio processor. At the time of writing, though, most USB speakers are still just basic stereo, and sound cards like Diamond's Monster Sound MX300 (reviewed here) do multi-speaker work well already. So unless you're really scratching for slots and system resources, you might as well stick with your standard sound card. I review a USB audio adaptor, which lets you use any amplified speakers as USB speakers, here.
Shielding
Magnetic shielding is important if your speakers are going to be hard up against your monitor. Since monitors use a magnetic field to aim their scanning electron beam, and speakers use large permanent magnets to give their electromagnetic voice coils something to push against, putting a magnet near a monitor can cause effects ranging from slight image skewing to serious colour distortion, which in extreme cases can only be cured by the use of a special degaussing wand.
Unfortunately, magnetic shielding generally involves using very small driver magnets, or sandwiching a reversed magnet onto the back of the main one, both of which strategies make the driver less efficient. Since magnetic fields fall off very rapidly with distance, you can use any old speaker as long as it's a reasonable distance from your screen.
My own "computer" speakers have monster 15 inch bass drivers with totally unshielded 1.5 kilogram magnets, and one of them is barely 60 centimetres from my monitor. The screen image is very, very slightly skewed anticlockwise, and that's the extent of the perturbation.
I knew I'd find a use for this picture one day.
You don't have to go as far over the top as me (by the way, my amplifier has four cooling fans). For not much money, you can get a whole lot more sound.
