PART 2: If your PC's hard drive is full to overflowing, then perhaps you may be tempted to expand sideways. In the last of this short series, MARTIN WALKER examines the implications of SCSI, and its application to the world of music. This is the last article in a two-part series. Read Part 1.

 

SCSI (or Small Computer System Interface) hard drives have always been a popular choice for hard disk recording, since they have consistently seemed to be slightly ahead in the speed stakes compared with IDE and EIDE drives (see the first part of this article in last month's SOS). The biggest advantage of introducing SCSI into your PC system is that it allows you to run devices outside the PC casing as well as internal devices, and this is especially useful if you are running short of internal drive space or also wish to use the SCSI device with a Macintosh or sampler (see my previous feature on integrating samplers into your PC setup via SCSI in SOS October '96). For high-performance applications, SCSI still leads over EIDE devices, as it offers true multitasking, allowing multiple drives to be handled simultaneously.

There are two main disadvantages to SCSI. Firstly, you need to purchase and have space inside your PC for a separate controller card (the so-called Host Adaptor). This contains the necessary circuitry to control the maximum of seven SCSI devices that can be used simultaneously. Secondly, the price of SCSI drives compared with IDE drives of the same capacity is significantly higher. Having taken the decision to 'go SCSI', the choice of a controller card is not without its problems. Adaptec cards are widely recommended as providing good performance and maximum compatibility with a wide range of hardware and software, but if you look in an Adaptec catalogue, there are literally dozens of different cards suitable for various machines and applications. In the search for maximum data throughput for the modern hard disk recording system, there are many factors that determine the maximum achievable speed. Most blanket recommendations mention the Adaptec AHA 1542 card as being an industry standard, since after selling millions of units it is the card that every manufacturer checks compatibility with, but even this has appeared in at least four flavours to date, and with a typical VAT-inclusive price not much under £200, it is surprising that so many people buy this card without considering the other options. At the other end of the scale, the Adaptec AVA 1505 card is less than £50, and may be perfectly adequate for basic applications.

MEET THINE HOST

The easiest way to narrow down the list of options is to exclude all but those suitable for the buss standard in your machine. Pentium machines have both ISA (16-bit) and PCI (32-bit) slots, so for optimum disk performance, always buy a PCI buss SCSI card if you have a Pentium. There are also VL-buss and EISA devices available, but these standards have largely been superseded by PCI. For those with a portable PC, SCSI controllers are available that use PCMCIA slots (Adaptec SlimSCSI) or which plug into the parallel port, but most of these have seriously restricted top speeds compared with normal cards, and whilst useful for connecting a Zip drive or CD-ROM, they are not really up to the rigours of hard disk recording. If you are considering buying a new computer, the other option is SCSI built into the motherboard. The chips often used in this case are made by Adaptec, and of course one more card slot will be available for other things. Overall, I think that SCSI motherboards may well be far more common in the future.

There are two more points to consider before narrowing down the field still further. If you propose to only use a SCSI drive with your PC, you will need booting capability on the SCSI controller so that it is possible to boot up your PC from the SCSI drive. If you already have a main drive conforming to the IDE or EIDE standards, you can continue to boot from this as before, but use the additional storage of your SCSI drive in tandem. For many people, this is an ideal way of working, as EIDE drives are still less expensive than an equivalent-sized SCSI one of similar performance, and by keeping your word processor, sequencer and other programs on this separate IDE drive, the SCSI drive will always be used solely for audio data, and can be regularly defragmented for optimum performance. This approach also makes it easier to use one of the new removable SCSI drives such as the Iomega 1Gb Jaz drive, or the SyQuest 1.3Gb SyJet drive, since individual projects of up to about an hour's duration can each remain on their own cartridge, whilst your PC can carry on running regardless using the separate main drive. If you decide to opt for SCSI booting capability, it is also sensible to ensure that your SCSI controller supports the addition of floppy drives, so that you don't have to fill yet another slot with a floppy controller just to add this one vital feature.

The most recent cards, such as the Adaptec 1542CP, are Plug and Play-compatible, which in theory means that no conscious choices have to be made concerning system resources needed by the card. When the board is installed, resources such as BIOS and port addresses are automatically assigned to prevent conflicts. Used with Plug and Play SCSI peripherals and cables, the AHA 1542CP adaptor automatically locates all connected SCSI devices, and instantly resolves any SCSI resource conflicts. It also automatically terminates the SCSI buss, eliminating the need for jumpers and termination resistors. This all sounds wonderful, but back in the real world, even Adaptec recommend that if using their card with any legacy (non-Plug and Play) devices -- such as virtually all soundcards -- it is safer to disable Plug and Play support on the SCSI card and configure it manually.

At the upper end of the market, there are Wide SCSI drives that use 16-bit SCSI busses (this is the connection between the SCSI card and the SCSI device, and is not to be confused with the buss used by the computer itself, which is 16-bit in the case of ISA card slots, and 32-bit for PCI slots). If you decide to buy one of these drives, you will need a Wide SCSI controller card to partner it. This will provide bandwidth capability of up to 20Mb/second for 16-bit devices, and will still support standard 8-bit SCSI devices, which can work at up to 10Mb/second.

MODES OF OPERATION

SCSI controllers use various means to transfer data. Most of the simpler ones now use PIO mode (Programmed Input/Output), as do EIDE drives. This mode uses a significant amount of processor overhead, as the processor has to move data to and from the controller. Incidentally, this is the reason why many people were caught out buying early eight-speed CD-ROM drives -- although data throughput is much higher at 1.2Mb/sec, the processor was tied up more than 50% of the time just reading data with some models, causing a bottleneck in the system. Simple SCSI cards using PIO will be fine for CD-ROM, CD-R, removable backup drives and a few tracks of hard disk recording.

The more advanced buss mastering mode uses our old friend DMA (Direct Memory Access) to carry out data transfer 'behind the scenes' without disturbing the main processor. Most higher-end SCSI controllers tend to use buss master DMA for highest performance. In general, choosing this type of card rather than PIO will give a far bigger increase in performance than upgrading from an ISA card using PIO to an equivalent PCI card. If you already use a Pentium PC and a soundcard with multiple DMA channels, do consider the higher-priced PCI cards, as using 32-bit slots will result in less demand on DMA than using 16-bit ISA slots (see the box 'Potential DMA Conflicts with Soundcards' for further details).

For slower SCSI devices, such as CD-ROMs and disk drives that handle up to about 2Mb/second, there will be little difference in overall performance between the different PC buss standards. With faster SCSI drives for hard disk recording, throughput will be significantly better if you use SCSI controllers on the PCI buss (the performance more than doubled for one user when they upgraded from an Adaptec 1542 ISA card to a PCI version, but results will vary widely depending on your system).

OLD WIVES' TALES

Buying a faster SCSI card will not make your existing drive go faster unless the SCSI card it replaces has previously caused a bottleneck. Even plain vanilla SCSI1 is capable of achieving 5Mb/second data transfer under ideal conditions, using short cables, synchronous transfer (see the 'Jargon Buster' box), and a fast PC. So, even with the cheapest SCSI card, you could carry out CD-R recording, data and sample backups with a Zip drive, or even simple hard disk recording on up to about four tracks, with few performance limitations caused by the card itself. However, cards using PIO mode (and also EIDE drives, which use the same type of transfer) give a significant processor overhead which, whilst fine in many other applications, may cause a bottleneck with higher-end PC hard disk recording systems. This is also why the latest EIDE drives, although having comparable speeds to their SCSI counterparts, can still limit overall performance, by tying up the processor when it could be replaying an extra audio track or three. The maximum speed of any PIO mode device will also be affected by the processor speed. A faster machine will thus tend to speed up the disk transfer rate, and this is one reason why drive manufacturer's speed figures can only ever be used as a guide.

For hard disk recording, not only the sequencer software design affects the attainable number of tracks, but also the efficiency of the digital audio drivers written by the soundcard manufacturer. All of these factors explain why the only valid way to measure overall disk drive performance for musical applications is to run a utility like HDSPEED.EXE (from SAW manufacturers IQS) or the Cubase performance utility (see my feature on PC hard disk recording requirements in SOS November 1996 for further details on these).

RECOMMENDATIONS

Although there are several other manufacturers of good SCSI cards, Adaptec is always the one that other people recommend. They have been in the marketplace a long time, have millions of units in the field with a huge range of types for every application, and are the first port of call for every SCSI device manufacturer when checking compatibility. Soundcard manufacturers also tend to use Adaptec chips for their onboard SCSI CD-ROM interfaces (for instance the Creative Labs SoundBlaster 16 SCSI). If you need a SCSI interface for a CD-ROM drive, do investigate this combined approach, but don't expect to add a high-speed SCSI drive to it. If you need a stand-alone SCSI card for CD-ROM, CD-R, or a lower-speed drive such as the Iomega Zip or SyQuest EZ135, then the AVA 1505, at a VAT-inclusive street price of about £45, will do the job adequately -- or with booting capability, the AVA 1515 model comes in at about £65.

For people on a limited budget, you will get reasonable throughput using one of the AHA 1520 series (AHA 1510, 1520, 1522), but the widest compatibility for different applications is still offered by the AHA 1540/2 cards, which are even more attractive in their new Plug and Play incarnations, and will provide much lower overhead using the buss master DMA transfer mode. Anyone with a Pentium and PCI buss is well advised to opt for one of the AHA 2940 series, which will cater for high-spec hard disk recording. However, don't bother to pay more for the 2940W or 2940UW types unless you plan to buy a Wide SCSI drive, since you will only get the higher performance with these models.

CONCLUSIONS

Ultimately, when using a SCSI device, the transfer speed will be determined by the weakest link in the chain, and for this reason, there is no point skimping on the controller card and causing a bottleneck. On the other hand, there is no point buying a high-spec device to run an eight-speed CD-ROM drive which transfers data at 1.2Mb/second, unless you need a high-spec AV system, which has to process video files simultaneously and requires all the processor time it can get. I needed a SCSI card to work with a SyQuest 270Mb external drive, and now use the AHA 1510A. This is perfectly adequate for the job, and I don't yet have the luxury of a PCI buss in my PC. If I were using a program like Cubase or Logic Audio, it would be more sensible to choose from the AHA 1542 range (or AHA 2940 range with a Pentium), but only if I could justify something like the new Jaz 1Gb removable drive. Otherwise, it would be still be cheaper to buy one of the latest large and fast EIDE drives, which have very similar performance to SCSI drives (albeit with a higher processor overhead), and add a lower-end SCSI card for use with external devices such as scanners and backup drives, which don't need such high throughput or have to talk to other SCSI devices such as samplers. However, one thing is for certain -- as our PCs fill up with more cards and drives, external SCSI devices will always offer a tantalising way of expanding sideways.

Using buss Master DMA can sometimes cause problems in audio-visual applications, since most full-duplex soundcards (ie. those that offer recording and playback simultaneously) will use two DMA channels, one each for recording and playback. With only three of the faster 16-bit DMA channels available on the PC (numbered 5, 6, and 7), conflicts can occur, and with a finite total time allocated for DMA, the SCSI card may hog too much DMA time, giving rise to audible dropouts with the soundcard. One solution to this problem is to use the lower priority DMA 7 for the SCSI card and select DMA 5 for playback and DMA 6 for recording. This will ensure that the soundcard gets all the time it needs for glitch-free operation.

Another option on some Adaptec controllers is to use the parameters 'buss On' and 'buss Off' to reduce the time that the SCSI card ties up the DMA buss. This can solve the problem, particularly if you are using the Digital Audio Card D Plus, but this may well reduce the maximum performance for SCSI. When DMA time starts to run short, it can also result in high-pitched whines appearing on the soundcard, or spurious faults. In some cases, you may even have to restrict your top SCSI speed to avoid audible problems.

However, rather than fighting for DMA time, it is well worth considering soundcards that don't use DMA. Turtle Beach soundcards, such as the well-respected Tahiti and Multisound, use a proprietary architecture named Hurricane which uses no DMA, and is claimed to be up to eight times faster than DMA transfer.

• SCSI1
This is the original version, with a clock speed of 5MHz, from which all subsequent standards have developed. It allows asynchronous transfers (see the 'Jargon Buster' box) at up to 1.5Mb/second, and synchronous transfers at up to 5.0Mb/second.

• SCSI2
A tweaked version of SCSI1, with slightly faster handshaking and a doubled clock speed of 10MHz. The result is that asynchronous transfers can run at up to 3.0Mb/second and synchronous transfers at up to 10.0Mb/second.

• FAST SCSI
Any device which can do synchronous transfers at speeds in excess of 5.0Mb/second. SCSI1 devices cannot manage this. Fast SCSI can achieve 10Mb/second. At still higher speeds, Differential SCSI (see the 'Jargon Buster' box) needs to be used for reliability.

• WIDE SCSI
This standard extends the buss width from its original 8-bit width to 16 or even 32 bits, with a corresponding increase in maximum speed to as much as 20Mb/second with 16-bit, and 40Mb/second with 32-bit busses. By its very nature, any device using Wide SCSI is also Fast, so the standard is sometimes also referred to as Fast & Wide SCSI. To take advantage of this, you need special Fast Wide SCSI2 drives.

• SCSI3
This specification is still being finalised, but adds UltraSCSI and support for fibre-optic and serial cables, as well as more devices on the buss.

• ULTRASCSI
A method that enables very fast data transfer rate on the SCSI buss, with maximum rates of 20Mb/second and 40Mb/second for Wide devices. Few devices currently support this standard.

There are various plugs and sockets in use for SCSI, making mail order purchases a bit of a nightmare unless you can exactly specify which connectors you require.

• 50-PIN CENTRONICS
The original SCSI1 used these connectors, which can be easily confused with parallel port leads. Don't be tempted to use a parallel port lead, though, however much cheaper it is. True SCSI cables use individually-shielded wires for each conductor, and are designed for high transfer rates (ie. high-frequency signals).

• 25-PIN 'D' TYPE
Many more recent SCSI1 and SCSI2 devices use 25-pin 'D'-type connectors, as seen on many Apple Macs.

• 50-PIN 'D' TYPE
Many SCSI2 devices use this type.

• 68-PIN 'D' TYPE
Mostly used by Wide SCSI devices, to accomodate the larger number of pins needed for 16-bit use.

• ASYNCHRONOUS TRANSFER
Asynchronous transfer uses an interlocked handshake, where one device cannot send more data to another until it receives positive acknowledgement that the other device has received the last data transmitted. This ensures reliability, but is slower than synchronous transfer.

• BUSS MASTERING
A high-performance method of data transfer in which the host adaptor's onboard processor handles the transfer of data directly to and from a computer's memory using DMA (Direct Memory Access) without intervention from the computer's microprocessor. This is the fastest method of data transfer available for multitasking operating systems.

• CABLE LENGTH
At the sort of transfer rates we all aspire to, the length of the cable has a significant effect on overall speed. Asynchronous transfer is faster on short cables, while synchronous is faster on long cables. Calculations show that in ideal conditions, asynchronous transfers in SCSI2 can achieve 6Mb/second with a one-foot cable, 3.5Mb/second with a six-metre cable, and 1.5Mb/second with a 25-metre cable. With typical cable lengths between one and two metres, 5Mb/second can normally be achieved. This why real transfer rates often seem to bear little relation to manufacturer's specs -- the latter are normally achieved with 'zero length' cables!

• DIFFERENTIAL SCSI
A balanced pair of wires is used for each signal, in the same way as a balanced mic cable. Susceptibility to noise and interference is greatly reduced, and longer cable lengths can be used.

• HOST ADAPTOR
The SCSI controller card itself.

• SCSI (SMALL COMPUTER SYSTEMS INTERFACE)
A buss interface standard that defines standard physical and electrical connections for devices. It enables many different kinds of devices (such as disk drives, CD-ROM drives, scanners, and tape drives) to interface with the host computer.

• SINGLE-ENDED (NORMAL) SCSI
A single wire in the cable for each signal that needs to be sent across the buss.

• SYNCHRONOUS TRANSFER
This type of transfer speeds up transfer by up to a factor of three, by sending multiple bytes before waiting for acknowledgement that the previous byte has been received. This mode was introduced to boost the performance on long cables. SCSI1 typically achieves rates of 5Mb/second with this mode, and SCSI2 has its maximum transfer rate limited to 10Mb/second.

• TERMINATION
A physical requirement of the SCSI buss. The first and last devices on the SCSI buss must have terminating resistors installed, and the devices in the middle of the buss must have terminating resistors removed. Failure to follow this will probably result in either unreliable data transfer or even SCSI devices that fail to work at all until termination is properly applied.

 

info

ADAPTEC
W http://adaptec.com

DAL (CARD D PLUS)
W http://www.digitalaudio.com

IOMEGA (ZIP AND JAZ DRIVES)
W http://www.iomega.com

IQS (SAW)
W http://www.iqsoft.com

SYQUEST (EZ AND SYJET DRIVES)
W http://www.syquest.com

TURTLE BEACH (SOUNDCARDS)
W http://www.tbeach.com

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