How do hdd recorders work




















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A hard disk recorder HDR is a high-quality digital direct-to-disk recorder system. An HDR is a digital audio or video multitrack recorder that directly transfers the live streaming data from the source to the memory.

Transfer rate, memory specification and processing speed all depend on the type of HDR being used. Recording time depends on the size of hard disk in the HDR. Software available for hard disk recording can convert any PC into a hard disk recorder, giving the user more flexibility and options to choose from. Audio recorded through an HDR can be edited non-linearly and can be accessed randomly, meaning that processing and editing is a lossless process.

By: Justin Stoltzfus Contributor, Reviewer. But there are disadvantages too. Because the technologies used in hard disk recording are so different from conventional audio recording, it is all too easy for the unwary to overlook the traps. So how do you find out about the other side of hard disk recording? Read on. Magnetic tapes follow a convention that is so obvious and subtle that you may never have even considered it, because it looks like plain common sense.

When you buy a tape, it has a length and a width: the length determines how long the tape will play for, whilst the width of the tape limits how many audio tracks of a given quality you can have. How long the tape will play for can be calculated by knowing the length of the tape and the speed at which it plays — although some tapes are commonly known by their time 'capacity' at a specific speed. Cassettes are an everyday example of this use of time to describe the length of a tape: a C90 lasts 90 minutes.

Calling a cassette a C90 is really a way of saying in short that the tape has enough length to provide 90 minutes of recording at the standard cassette tape speed. When we're talking about magnetic tapes, their capacity can be expressed using their recording time and the number of tracks they provide. The reason is that the tracks are all tied together physically — when you play the tape, the tracks all play simultaneously.

You can't take the tape and play it continuously for eight hours — at least, not without stopping it and reversing direction, or rewinding! But for hard disk recording, this 'obvious' relationship between tracks and time does not exist in anything like the same form.

The reason this figure is sometimes quoted for hard disk recorders is because it can be easily determined from the size of the hard disk and the data rate of the digital audio. The 'record time' calculation represents the idealised total capacity of the hard disk to store mono digital audio. As the number of tracks increases, so the time goes down to match. Stereo audio requires two tracks, so the capacity of a 1Gb drive is just under 95 minutes. For four tracks, the calculations give a capacity of just 47 minutes.

In order to emphasise this correspondence between time and number of tracks, the term 'track minutes' is being adopted by some manufacturers as a way of describing the capacity of a hard disk recorder. A 'track minute' is exactly what it says: a minute's worth of recording on one track. So a machine which has a capacity of 60 track minutes will be able to record 60 minutes on one track, or 30 minutes on two tracks, or 15 minutes on four tracks, or even 7. It's easy to expect a disk recording system to be able to do everything a tape recorder can do, and more.

But disk recorders have problems all their own, and you need to be aware of what can go wrong. One very significant difference between multitrack tape recorders and disk recorders is that if a tape recorder claims to have eight tracks, it will always give you eight tracks — at least while it's still working properly. But the number of segments that can be replayed from a hard disk is not a constant.

Some will allow their systems to reproduce as many simultaneous segments as it can until the limit is reached, when alarm bells will ring.

Others advertise their systems as being capable of a certain number of simultaneous segments, knowing that a user will always get that number, except perhaps in extreme circumstances. Bear in mind that the rest of the system has to be as fast as the disk to achieve this, and if the audio has to go through the processor of a standard personal computer this may not always be the case. If the audio was originally recorded on the disk in long continuous segments, and you haven't done much editing, then playing back this audio will be easy.

But if you have edited the audio heavily into short segments that occur frequently, and rarely in the order of the original recording, then the mechanism of the disk may not be fast enough to maintain the required data flow. This is the big question, and my answer at this stage and in the more affordable price bracket is no. If I had been able to answer yes to this question, I wouldn't have invested in a Fostex RD8 digital multitrack recorder, which I hope will see me through to the time when a hard or optical disk recorder is a total replacement for tape.

The one thing that a tape recorder has, digital or analogue, which disk recording systems do not, is simplicity. I can connect a simple remote control to the Fostex, as I did with my previous analogue multitrack, and produce and engineer a vocal session with my fingers on just five transport keys. Operating the machinery takes approximately 0. I have tried to do the same using a disk recording system, and although it's difficult to put a figure on the amount of brain capacity it uses, there certainly isn't enough left over to get a decent performance out of the singer.

Tape recording can be done by touch, and by almost subconsciously hearing the whirring of the machine as it changes between transport modes. If this delay varies, you're in big trouble.

There are moves towards compatibility between disk recording systems, but it's not commonplace yet. In conclusion, I would say that the time is ripe to move into disk recording, as an addition to your existing equipment but not as a replacement. Don't hang back waiting for the perfect system to arrive because great benefits are available now, and the more of us who join the party, the more fun and excitement there's going to be!

Most hard disk drives were never intended for audio or video use, but rather for routine data processing where short, random bursts of data are the norm. These drives use gaps in the data stream to carry out calibration and error correction tasks that can take up to half a second to perform. This is obviously a serious drawback for audio and video, where a continuous flow of data must be maintained. The problem can be alleviated by storing data in a buffer memory so that there is always data available when the disk chooses to recalibrate itself to compensate for thermal expansion, as it will without reference to the host computer.

Even so, the risk of missing an audio segment or playback stopping is increased, because the disk is not inherently capable of delivering data in the continuous way required.

Fortunately, one disk manufacturer has wised up to audio and produces a range of disks specifically for audio and video requirements.

These disks have been optimised to produce a high sustained data rate, and although it is still a good idea to consult the manufacturer of the disk recording system, they are likely to produce more reliable results than disks which were never intended for audio.

I recently installed a Micropolis AV 1. The only drawback of this disk is that its mechanical noise is higher pitched than my other drive and rather annoying, but if your disk recording system uses SCSI drives and the manufacturer gives you the OK, a Micropolis AV drive is recommended at the moment.

Something you need to consider when choosing a disk recorder is the number of physical outputs it has. Analogue and digital tape recorders always have one output per track, but if a disk recorder is capable of playing eight simultaneous segments, that doesn't mean it has eight outputs. It may only have four, and you will have to allocate more than one sound to each output.

This imposes limitations on any EQ and processing you may want to provide during mixdown. At the time when I was looking at suitable drives, I only intended to use a disk system for stereo editing and for assembling masters.

The maximum running time of a CD is about 80 minutes, so my first thought was that I needed an Mb drive. Then I realised that I would need to record extra material to edit down to this length, so a safety margin would be necessary — 1Gb seemed about right. Then I thought about my other recording activities, which involve recording classical concerts.

But since the DAT I use for location recording only samples at 48kHz from the analogue inputs, this takes more disk space — so now I'm up to 1.

I nearly made the mistake of buying a disk this size, but I then realised that formatting and partitioning reduces the capacity, so I had to buy a disk which was nominally 1. Optical disks are very similar to hard disks and can be used for disk recording in many cases. Their great advantage is that they are removable, so it is possible to afford many more megabytes than you can using fixed hard drives.

The disadvantage is that the optical mechanism is heavier than the pickup of a magnetic disk and therefore access time and data rate are not as good. I use a Sony optical disk which can store Mb per side and is just capable of stereo recording with my system on a good day.

It can play back a stereo recording on any day, so I usually record onto my hard disk and copy onto the optical for storage. It just isn't fast enough. Having said this, there are new, larger optical drives coming onto the market which may be viable for multitrack disk recording. This is something I intend to check out in the near future.



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