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IDE or SCSI ?

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When a musican wants to buy a computer, he has to face many choices. If you want to use Cubase VST as your recording audio system, you have to choose between Windows (98/2000/XP) and MacOS. If you choose Bill Gates side, which processor do you choose, Intel, AMD, and for these two choices, what might be the speed processor, the amount of RAM, the audio card (pro or multimedia), and so one... One of the most difficult choices to be done is which type of HD controler I have to take, IDE or SCSI, or even a mix of both system ? It is commonly admit that IDE is the cheaper solution and SCSI the fastest and the more performant. But what are these to interface, in which points are they different ?


What is the drive interface ?

The drive interface, is, in a few words, the middle that allow communication between computer and its different kind of drive such as CD-ROM, Hard Drive (HD), Zip reader and other drive having a specific communication protocol. When a musician record audio in multi-track, he needs fast HD and an interface letting him transfer data very quickly, especially there is a huge quantity a audio track playing together at the same moment. A mono track, 16 bits, 44.1Hz, need, to be correctly played, a transfer rate of 75 Kbits per second approximatively. Seems to be quite few, but when in a song, you have more than 16 audio tracks and you record 4 or more audio tracks simultaneously, the HD performance and its interface make the difference between a computer who can handle that and the one who will fail for such a peace of cake.


IDE

IDE means : Integrated Drive electronics. It's also called ATA for Advanced Technology Attachment. The advantage of this interface is to be always there on the motherboard of your PC and even on a MacG3 station. This interface is the most frequently meet interface du to the fact that it is very easy to engineered and mainly du to lost cost of the peripherals.

The drawback of this technology is the it remained, for a long time, processor dependant. But this has changed with the apparition of new standard call Ultra ATA or DMA. The U-ATA use the PCI bus and manages some tasks linked to the peripherals managment. This represents a big step with regard of the IDE system. Some benchmarks relate that the processor use is about 5/10% for U-ATA when he reaches 60/70% for IDE.

U-ATA technology also incresed by two the data transfer rate.. E-IDE or IDE has a transfer rate of 16.6Mo/s at last, when U-ATA can be 33.3Mo/s, or 66.6Mo/s for the new version of the standard. Moreover, many HD constructors offer U-ATA models with 7200 RPM (Rotation speed) which let you reach transfer rate that was reachable only by using SCSI HD.

However, IDE technology supports only a max of 4 internal peripherals shared by 2 channels.In this way, you have two peripherals by channel, a master and a slave.Each channel needs an IRQ by computer, which can limits the extensions of the computer. This forces the IDE port to treat only one instruction at the same time. Being able to execute only one operation at a time can restrain the system performance with application where the time-access to data are very critical. Such application like Music or video on computer are in this case.


SCSI

However, IDE technology supports only a max of 4 internal peripherals shared by 2 channels.In this way, you have two peripherals by channel, a master and a slave.Each channel needs an IRQ by computer, which can limits the extensions of the computer. This forces the IDE port to treat only one instruction at the same time. Being able to execute only one operation at a time can restrain the system performance with application where the time-access to data are very critical. Such application like Music or video on computer are in this case.

SCSI is processor independant interface, between computer and intellignet periphericals (HD, CD-Rom, floppy disk, scanner)... These periphercials are called "intelligent" because they can handle many instructions at the same time by optimisation, without requiring the CPU. A SCSI HD can receive many instructions as the same time. It will optimize the execution order to minimize head movement and maximse data writting.SCSI periphericals are linked together using serial port and own a Identification number which determines their priority, and their weight in the chain.

It's common to call Host SCSI interface SCSI controler but this is a mistake. SCSI are only interfaces which translate computer requests into comprehensive informations for SCSI periphericals. Each of them has its own SCSI integrated controler. This is the reason why it's so called intelligent peripherical.

There are two modes for SCSI communication : synchronous and asynchronous mode.Asynchronous mode ask for aknowledge for every request when synchronous mode doesn't. Synchronous mode is muc more faster than asynchronous one.

There are many SCSI standard but these have high compatibility level betwen them. Two types of connections can be identified : narrow or wide. SCSI narrow connection allows 8 bits parallel transfer (with 50 pins connectors). Seven periphericals can be chained with SCSI narrow. SCSI Wide connection deals with 16 bits parallel transfer thru 68 pins connectors and can be chained up to 15 periphericals. A factor x2 exists between narrow and wide.Here is a short view of the standards

STANDARD SCSI Other name Taux de transfert maximum Narrow SCSI Max transfer Rate Wide SCSI
SCSI - 1   5 Mo/s  
SCSI - 2 Fast-SCSI 10 Mo/s 20 Mo/s
Ultra SCSI Fast-20 20 Mo/s 40 Mo/s
Ultra 2 SCSI LVD 40 /s 80 Mo /s


SCSI is a very fast interface where a connected intelligent periphericals able to managed many tasks without using CPU.Moveover, new delopment are based on this technology. Last produced HD or storage units (the fastest) are often based on SCSI far before IDE or parallel port. For instance, IDE HD at 7200 RPM are available for 1 year when it was available for 3 years with SCSI. For SCSI, it's now about 10000 RPM and soon 12000RPM. At last, SCSI minimize the use of the processor in periphericals management but it also use only one IRZ for an host interface.

A point against SCSI, it is a little bit harder to install. We must check that every peripherical has different ID, that, the total length of the wire is less than 3 meters (excepter LVD, 12 meters) and also that the chain extremity has a ending.Moveover, it is very easy to make a confusion between the needed wire du to a vast quantity of standards. But the main drawback is still the price.To install SCSI into your computer, you have to buy a SCSI host interface card or a motherboard with one. This cost a lot. SCSI periphericals are mostly more expensive than the same with IDE or parallel port. For instance, a SCSI HD is double price than a IDE HD for the same storage capacity. Even adaptators or wire are sold at higher prices.


Other affordable interfaces

We can't forget two other interfaces : USB and FireWire. USB (Universal Soldier Bus, sorry Serial Bus) stands to be a "plug and play" interface for low bandwidth periphericals (printer, mouse, Zip). In theory, you can connect with this bus up to 127 periphericals at the same time. You can even connect a USB peripherical without shuting down your computer. However, USB bus offers a poor bandwith (max 12Mo/s) and is mostly used for low speed periphericals

The FireWire or IEEE1394 is a new interface with all the advantage of the USB plus a big bandwidth ( 100 to 400 Mo/s). This interface will replace in the futur SCSI and IDE. Only Apple offers it on its Workstation. But intel doesn't want to put it into its integrated chipset for motherboard. This delays a lot the use of the very attractive interface

For the moment, only two real choices can be made : SCSI or IDE.


How to make a choice ?

It depends on what you wanna do. SCSI is great for its capacity to handle many requests at the same time without consuming CPU Time.With Cubase VST, many users gonna have songs with a lot of audio tracks being played together. If a musician mainly uses Midi functions, or doesn't use more than 12 audio tracks at the same time, a U-ATA interface will be fine. This also provides more CPU Time for handling effects and eq-in put on audio tracks. But, as U-ATA only handles one instruction at a time, it can fastly lead to make a bottleneck for a computer having to played more than 20 audio tracks. In cubase VST, data flow are very critical. IDE HD is like a single way highway. Even if you can drive fast, too many drivers will reduce the flow. SCSI is like a many ways highways where the flow is managed, able to deal with a denser circulation and able to minimize bottleneck risks.As everybody knows, when Cubase can't get the all audio information on time, crackles appear during recording or playback. So, If you are a greal dealer of audio tracks, SCSI is really for you. But be careful... you'd better get a fast IDE HD than a slow SCSI HD. SCSI HD performance are based on design, electronics, mechanism... this is why it is so expensive.

Another advatange for SCSI is its expansion capacity. Knowing that audio files are disk space consuming, it is very interresing to add some periphericals if needed, very usefull when you take into account that it is better to use 2 HD for audio recording. For a professional studio, the fact that SCSI only takes one IRQ, gives you the possibility to add many professionnal cards, one for MIDI, one for audio and maybe a second graphic card to work with multi screen environment. You can have SCSI and IDE cards on your computer. That's what the majority of us does. Even if this system is very popular and cheap, it consumes a lot a ressource of your computer and limit the extansion capacity for cards (graphicals cards, audio cards)...

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Martin BOYER, on the 18-02-1999

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