Alesis ADAT HD24
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The Alesis HD24 is a 24-track hard-disk audio recorder. It is the successor to the hugely successful tape based Alesis ADAT.
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[edit] Advantages over tape-based ADAT
There are numerous advantages that the HD24 has over the tape-based ADATs. Primarily, the use of hard disks as storage medium rather than tape allows for random access of audio, whereas tape based solutions only allow sequential playback.
The built in Ethernet port allows users to transfer the audio files to a computer, which could only be done with a specific PCI card with the previous ADAT. This feature permits users to transfer the audio to a Digital Audio Workstation for editing, after which the audio can be transferred back to the HD24. However, transfer speed of the Ethernet port is limited to 10 Megabit per second.
[edit] Advantages and disadvantages compared to computer-based recording
Although it is possible to edit audio within the HD24, computer based systems offer a lot more editing power. The additional benefits of computer based recording mean that the HD24, or any stand alone multi tracker, is now not the preferred choice of many audio engineers. However stand-alone solutions are often perceived as being more reliable than computer based solutions.
[edit] File storage and transfer
Alesis developed their own method of writing to disk which interlaces the audio of the various tracks on disk. Although this does not entirely resolve fragmentation issues, it greatly reduces them. This prevents many seek operations, allowing the HD24 to use cheap IDE drives for 24 track audio storage.
Disks formatted with Alesis' proprietary FST file system are not currently recognized by today's operating systems. Therefore, to permit interoperability between computers and the Alesis ADAT HD24, the HD24 recorder is equipped with a built-in FTP server. This does not require any additional hardware, but is generally considered slow, and many users have experienced dropped connections (which has been attributed to expired DHCP lease).
The combination of low transfer rates and dropped connections of the built-in FTP server has driven many HD24 users to opt for an external file transfer solution. The available options are the Alesis Fireport in combination with Alesis' FST/Connect software, or an HD24-compatible drive bay with the third-party HD24tools suite. Either combination allows for file transfers with a theoretical transfer rate of 400 megabit per second. However, in reality, this rate is seldom achieved as the throughput of the hard drive itself can be a limiting factor.
[edit] Compatibility
The Alesis ADAT HD24 offers good out-of-the-box compatibility, both with existing ADAT systems and PC-based solutions.
The HD24 has MIDI in and out and can transmit MTC (Midi Time Code), which allows for easy synchronization to other hardware, however it cannot synchronize to incoming MTC. The HD24 supports MMC (Midi Machine Control) in two directions. It also features three Alesis Lightpipe ins and outs (tape based had one of each), which each can transfer eight tracks of digital audio at once.
The HD24 also has 9 pin synchronization ports, dedicated to sync with other ADAT machines. An HD24 can easily be integrated into an existing tape based ADAT system, although there are a few issues with compatibility between the two; for instance, by default HD24s start each song from zero (although the zero-offset can be moved), whereas the tape based ADATs continue to the end of the tape no matter how many songs are recorded.
The built-in FTP server permits transferring files to and from computer-based Audio Workstations.
The connector-side of the original Alesis HD24 drive caddies reveals that they were designed by a company called ViPowER. Users on the HD24 Yahoo forums have confirmed that the Alesis HD24 caddies are interchangeable with ViPowER caddies, as well as with some no-name brands.
It should be noted that Alesis' FST/Connect fireport software only supports drives that are connected via FireWire and will therefore not work with USB drive bays. Even firewire versions of alternative drivebays have yet to be confirmed to work with the original FST/Connect software.
The HD24 was originally designed to work with IDE (PATA) drives. Although natively it does not support SATA drives, the HD24 community on Yahoo groups have determined that the caddies can be fitted with a PATA-to-SATA conversion board. This seems to work reliably with boards employing the Marvell (88SA)8040 chip, whereas the drive is not recognized when using conversion boards that use the JM20330 chip. This caddy modification extends the effective lifespan of the HD24 beyond the availability of IDE drives.
[edit] Internal
The HD24 operates on a Nucleus embedded real-time operating system, running on a Motorola MCF5206 EFT54 processor, which combines a Coldfire processor core with several peripheral functions such as a DRAM controller, timers, parallel and serial interfaces.
A 26-pin port on the mainboard labelled 'Motorola BDM Port' permits background debugging using a Background Debug Mode interface adaptor.
There are two IC41LV16256 chips in the HD24: according to the data sheets, they are 256kx16 bit Dynamic RAM with EDO page mode, giving the HD24 a total of 1 megabyte of RAM.
The Ethernet chip is a CrystalLan CS8900A-CQ3, a 10Base-T LAN chip.
For recording, the original HD24 uses 12 stereo Wavefront AL1101G A/D converters, and for playback it uses 12 stereo AL1201G D/A converters. The HD24XR model uses the higher-quality AKM 5393 A/D and AKM 4393 D/A converters.
The HD24 is equipped with an Analog Devices ADSP 21065L KS-264 digital signal processor. There is an ALTERA ACEX EP1K30QC208-2 FPGA (Field Programmable Gate Array) in there as well.
The HD24 little remote control (LRC) is almost identical to older ADAT LRC's. The buttons are labeled differently, but internally the remote controls are identical: a simple resistor network.
[edit] Known issues and how to deal with them
FTP transfers are known to be slower than real-time and not very reliable; this has been attributed to lost DHCP lease. Both of these issues can be overcome by obtaining an external transfer solution.
Power outages during recording may result in lost recordings, leaving the drive in a corrupt state. This is due to the allocation information of a song only being written to disk when STOP is pressed. The power loss problem can be addressed by powering the HD24 recorder via a UPS. Alternatively, one can pre-record silence with the expected length of the recording or longer. Often, it is possible to recover the audio that was lost due to power outages.
The HD24 stores adjacent tracks in adjacent sectors. This guarantees minimal drive head movement across the platters of the drive, but fragmentation in the time domain can still occur. As there is a limit on the number of fragments that a song can contain, heavily fragmented drives may cause data loss. This can be prevented by always recording to a freshly (quick)formatted drive. This best practice will also make it easier to recover from power outages.
Hard drives that are kept on a shelf unused for a long period of time may stop functioning and no longer spin up. This is due to the lubricant of the drives setting, and not a problem inherent to the Alesis ADAT HD24 recorder itself. The problem can be overcome by regularly spinning up the drives and by making regular backups.
The Macintosh version of the FST Connect software has only been released for the PowerPC architecture and not universal binary to support the Intel architecture. This was not an issue until Apple released the 10.4.9 update to its operating system.
Some users have reported the HD24 recorder to fall out of record mode when used in extremely noisy environments due to sonic vibrations. This has reportedly been overcome in several ways, among others fixing the hard drive to the chassis of the recorder; placing the recorder on a soft surface (such as a blanket); or simply shielding the recorder from the loud sound. A solution that was reported successful by HD24 users on Yahoo Groups is to insert washers (thin metal rings) under the connector screws. This will slightly push out the connector, allowing it to make better contact.
There is a possible design flaw in the HD24 where the connector is firmly attached by the two screws leaving no tolerance, which means that the caddy remains in tension and can pop out spontaneously or as a result of vibration. It is possible to work around this by loosening the two retaining screws for the D type connector at the back of the disk caddy. See picture.
The IDE connector inside the caddies does not always exactly fit with the IDE connector of the drive. Symptoms are a constantly-lit LED on hot-swappable ViPower drive bay, slow booting and the drive not being recognized, either by the HD24 or by the computer. If this occurs, swapping the drive to another caddy may help.
[edit] Support
The HD24 forum on yahoo groups is the prime source of support for the Alesis ADAT HD24 recorder. This group is also monitored by Alesis. HD24@yahoogroups.com
