PART 2: Brian Heywood concludes his look at PC soundcards by considering some of the issues involved in using them to record and play back digital audio. This is the last article in a two‑part series.
Last month, I looked at the various types of synthesis offered by different PC soundcards; this month, I'm looking at how to generate sound from your card in a different way, using the digital audio playback and record capabilities offered by some MPC soundcards.
MIDI VS. Digital Audio
Digital audio playback via your soundcard is actually conceptually simpler than MIDI synthesis, in that your PC is simply being used as a glorified playback unit. In this case, the audio is stored digitally on your PC's hard disk, and then converted to an analogue signal suitable for amplification by the circuitry on the soundcard. You are far more likely to hear digital audio playback than synthesis from your trusty PC in multimedia applications, for two reasons: firstly, the quality of playback is far more predictable; and secondly, it is easier to produce an audio file than a MIDI file, since the latter requires a certain amount of musical skill. In fact, if you want to use your MIDI system to produce a multimedia soundtrack, you may end up mixing it down into the PC's .WAV digital audio file format to ensure that quality is maintained across the wide range of available soundcards, rather than creating a MIDI file which may seem of lower quality when reproduced using the sounds on an inferior card.
From a technical standpoint, digital audio playback is simpler than synthesis as well, since all the soundcard has to do is to turn a digital number into an a voltage, rather than perform the mathematics required for true synthesis, or the polyphonic scanning of memory required for wavetable playback. In some ways, recording digital audio is also technically simpler for the PC than recording MIDI: with digital audio, the PC hardware has to scan the audio input on a regular basis (ie. at the sample rate, for example 22.05kHz) and transfer the information to disk. This happens regardless of whether there is any signal present or not, and silence takes up as much space on your hard disk as an audio signal. A MIDI performance, on the other hand, is far more unpredictable. MIDI data will not arrive at the PC at all in quiet passages, and then can arrive thick and fast in note‑heavy passages utilising plenty of controllers. This means that interrupt handlers need to be used (see the 'PC Concepts Explained' box elsewhere in this article).
Sounds Fine, But...
Like the synthesizer section of a soundcard, the digital audio playback section needs to have a driver that allows Windows audio applications to access the audio hardware using the standard Windows Applications Programming Interface (API). While digital audio playback and recording is conceptually simpler than MIDI, this driver does have to be well written in order to cope with the sheer amount of data generated to reproduce audio digitally, and to cope with the constant, relentless nature of the process. When you purchase your soundcard, it is important to make sure that you find out where (and if) such drivers are available.
The only troublesome technical aspect of digital audio playback (as opposed to recording) is the large amount of data that has to be transferred between the disk and the digital‑to‑analogue converters. When sampling at the oft‑quoted 'CD quality' (16‑bit stereo at 44.1kHz, but more on this in a minute), the PC has to shift just over 10Mb of data across the data buss every minute, or 175K per second. This might not sound a lot in computer terms — after all, even the original IBM PC's data buss ran at 4.7MHz. However, audio data has to cross the buss twice — it is read from the converters into RAM, and then written from RAM to the hard disk. It also has to compete with other data that wants to use the buss, like graphics (and other) information read from the hard disk. Unlike this other data, though, audio data can't wait if the data buss becomes momentarily clogged; if the data doesn't get through, it's lost for ever (if you're recording) or appears as stuttering (on playback). Martin Walker's article on PC hard disk recording requirements in the PC Musician section of last month's SOS gives some useful tips on how powerful a PC needs to be to play back digital audio reliably, although the software you use can also have an effect on how many tracks you will have available — take a look at the same article for some useful tips on how to optimise your PC's performance for digital audio.
CD Or Seedy Quality?
If you're planning on using a soundcard for musical applications, the quality of the card's audio output is obviously going to be important, and you may encounter many cards claiming to offer 'CD quality' output. This is a bit like seeing a second‑hand car dealer called 'Honest John's', or believing that a politician will keep their pre‑election pledges after they get into office. The 'CD quality' label usually simply refers to the ability to play back 16‑bit stereo sound sampled at 44.1kHz, and doesn't say anything about the quality of the analogue circuitry between the converters and the external connectors. As many soundcards are targeted at the games market, they have to be extremely competitive on price, and corners are often cut on the analogue circuitry. I've heard reports that the least significant two or three bits on cheaper cards are simply random numbers (ie. noise), which reduces the effective sample size to 14 or even 13 bits. Certainly, some consumer CD players have better audio characteristics than the majority of soundcards on the market. However, there are a number of cards on the market that can produce true CD quality. Cards like the Gravis UltraSound Max, the Roland RAP10, the more expensive Turtle Beach cards and the Terratec Maestro 32/96 card (mentioned in October's PC Notes) all have nice clean audio outputs. Of course, it depends on what you want from your system; the SoundBlaster AWE32, for example, is perfectly adequate for producing demos, and many other cards fall into this category.
Apart from the quality of the analogue circuitry, there are other factors that affect the quality of a soundcard's audio output, the most important being the quality of your PC's power supply, which was probably not designed with audio in mind. Some cards take this into account, and heavily filter the power lines on the card, but this adds cost to the card. Another potential problem that occurs when you have audio electronics inside the PC's case is that of radio frequency interference (RFI) between adjacent expansion cards. This can be reduced by keeping video and hard disk controller cards away from the soundcard. You may have to try various configurations before you find the best arrangement.
Synchronicity
Aside from the audio functionality of your soundcard's digital audio section, another point to consider is whether it has a full or half duplex capability — ie. whether or not you can play back and record audio simultaneously. This might not seem too important, but consider how difficult it could be to record a backing vocal without being able to hear the original (I know, I've tried it!). Martin Walker pointed out last month that you could record in 'sync' using a MIDI backing track for reference, but this falls down if you need to record a track that fits with one that is already on disk. To perform this task, a full duplex card needs to have two independent access channels — usually DMA channels — and this takes up twice the buss resources that a half‑duplex card requires. Such added complexity makes the card more expensive to manufacture, the software driver more expensive to develop, and installation more problematic — so it's perhaps not surprising that most games soundcards don't bother with full duplex operation. Still, there are a reasonable number of cards available with this facility. Cards from Gravis, Roland and Turtle Beach have had it for quite a while, and the SoundBlaster AWE32 driver has recently been updated to provide full duplex operation (call 01734 344744 if you need a driver update). Again, it's always worth asking when you buy a card whether it has this capability.
The Digital Connection
Until now, we have only considered cards that play back samples as an analogue audio signal. There are situations that you might want to transfer the audio direct to or from other digital media (like DAT), while remaining in the digital domain. Alternatively, you might want to use external converters to side‑step the interference problems you can encounter with analogue audio circuitry inside the PC. I've only encountered two digital cards: Creamware's TripleDAT and DAL's CardD. These cards have S/PDIF (Sony/Philips Digital InterFace) connectors that allow the digital information to be directly transferred to and from the PC, with none of the possible signal degradation that can occur when digital audio is converted into an analogue signal, sent to another digital medium, and reconverted to a digital signal there. The AWE32 card is curious in that you can take the output from the onboard synth as a S/PDIF signal, but not digital audio. More cards with digital outputs are due on the market in the next few months; the aforementioned new card from Terratec will have two digital outputs and one digital input, and there have been persistent rumours that a new Yamaha card will have digital ins and outs.
The Bottom Line
A computer with a soundcard, even if it has a modest digital audio replay section, can act as a very compact and easy‑to‑use music workstation. If you are producing demos, or using your computer as a kind of musical notepad, then the digital audio section of a soundcard can add a lot of impact, even if it's only used to add a vocal line. However, bear in mind that it is very easy to lose sight of your objectives when delving into the technical minutiae of audio equipment. There is not a lot of point insisting on high‑quality audio — say a 90dB noise floor — unless you are planning to produce commercial‑quality CDs on your PC. It just depends on your ultimate objectives.
Disk Space Requirements For Digital Audio Data (MB/Minute)
| SAMPLE RATE | 11.025kHz | 22.05kHz | 44.1kHz |
| BIT RESOLUTION | |||
| 8‑bit mono | 0.66Mb | 1.3Mb | 2.6Mb |
| 8‑bit stereo | 1.3Mb | 2.6Mb | 5.25Mb |
| 16‑bit mono | 1.3Mb | 2.6Mb | 5.25Mb |
| 16‑bit stereo | 2.6Mb | 5.25Mb | 10.5Mb |
The Wave Of The Future?
Last month, in his article on PC desktop music systems, Paul White mentioned software synthesizers that use the digital audio outputs of your PC to output waveforms generated by the program. There is of course nothing new about this — tracker applications have been doing it with MOD files for ages. A MOD file uses short samples to produce sounds, transposing them on the fly to create tunes. Many games soundtracks have been created using this method, but what is now new is the ability to control the software synthesizer in real time via MIDI; although, as Paul pointed out, this process does require a lot of processing power. The most successful software synth to date comes from Yamaha, but you need at least a Pentium PC to be able to take advantage of this technology.
More PC Concepts Explained In Brief
Many of the terms used in this month's article were clarified in the 'PC Concepts Explained' box in last month's instalment of this article. Many others are also explained in Martin Walker's article on configuring soundcards, which starts on page 164 this month.
- MIDI INTO DIGITAL AUDIO
One question often asked is how a MIDI file can be turned into an audio file on your disk. It would be a relatively simple task for one of the software synthesis packages to do this (at least compared to creating a MIDI file from a mixed audio file — the data just needs to be written to disk rather than to the digital‑to‑analogue converters), but at present, I don't know of any application that performs this task. So, at the moment, the only way to do this is to loop your soundcard's synthesizer output to its line input, and record it that way. - DSP (DIGITAL SIGNAL PROCESSOR)
This is a computer processor specifically designed to handle the type of maths required for processing sampled analogue signals. Originally developed for radar and radio signal processing, they have been applied with great effect to digital audio signals; in fact, you find some sort of DSP in every type of digital effects units. A soundcard equipped with a DSP can make use of it to relieve the load on the PC's main processor when playing back digital audio. - IRQ (INTERRUPT REQUEST) LINES
The Interrupt lines allow PC hardware to alert the processor chip that a high‑priority event has occurred and needs attention. This means that unpredictable events — like MIDI bytes from a live performance — can be read by the PC without tying it up completely. Theoretically, the IRQs can be shared between a number of soundcards, but in practice this almost never happens.. - PULSE CODE MODULATION (PCM)
The protocol used by Windows to encode the audio data into the .WAV file format. This format can handle a wide range of sample rates, bit resolutions and number of audio channels. There is also a variant protocol called ADPCM that compresses the audio data, thus reducing the size of the file.