Re: [linux-audio-user] Understanding HDD capabilities.

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Subject: Re: [linux-audio-user] Understanding HDD capabilities.
From: Patrick Shirkey (pshirkey_AT_boosthardware.com)
Date: Tue Dec 04 2001 - 16:12:26 EET


Jörn Nettingsmeier wrote:
>
>
> 108.8M / 768K = 145 mono streams max.
>
> but i think this value is purely theoretical. the moment the drive
> needs to move heads, you're lost.
>
> iirc, paul d. claims he can do 32 tracks i/o, but that's on a mean
> smp scsi-only system. 
>

Guenters explanation hit home.

But this page on the net has an even better idea of it. Only I have no
idea how to get the true physical disc geometry.

http://www.pcguide.com/ref/hdd/perf/perf/spec/transSTR-c.html

What would be useful is to have a complete breakdown of the bottlenecks
for recording digital. I'll attempt to start a list if others want to
add that would be good.

1: A preamp which you can plug your gear into before sending to your
sound card. Going direct into a sound card produces dubious qualtiy
recordings but routing through a preamp adds a certain warmth to the
sound which is hard to reproduce digitally.

2: Sound card - The more inputs it has the more seperate tracks you can
record . Use the highest sampling rate your card can achieve. This is
measured in Hz and is the amount of times per second that your card
takes an audio snapshot of the stream. Use the highest bit rate your
card can achieve. Cd quality is 16 bit (2 byte) 44100 HZ, stereo but cds
are old technology so if you want your music to last the distance try
for more. 24 bit is directly related to the pain threshhold limit of the
human ear. Down sampling (24 -> 16) is possible but up sampling (16 ->
24) is not.

4: Modern CPU's can process multiple tracks. If you have anything over
233mhz then you should be ok unless you plan to do fx processing at the
same time.

5:If you intend to do fx processing while recording or buffering then
you will need a fair chunk of memory. Anything over 128 MB is good but
try to get more than 256MB if you can. Ram currently comes at three
speeds:
         SDRAM = 133mhz
        DDRRAM = 266Mhz
        RDRAM = 800Mhz (pIV)

6: ATA/ide transfer bus works at 66 MB/sec. This will allow transfer of
around 500 mono tracks but you would need a very powerful cpu to process
that many tracks without sound dropouts.

7: The Sustained Transfer Rate (STR) and Internal Media transfer Rate
(MTR). Both of these are the bottleneck for your music once it is past
the Analog->Digital converters on your sound card. The STR is the amount
of MB/sec that your HDD can write to a non fragmented area of your HDD.
ie the data is placed in one area not spread around the discs. The MTR
is the amount of MB/sec that your HDD can write to a single track on
your disc. This number is usually totally misleading because it doesn't
take into account the time it takes to move the heads in the HDD, to
spin the disc or switch between tracks. It is primarily a "theoretical"
specification that illustrates the state of the drive's technology.

There is a way to find the possiible STR of your drive by just using a
calculator.

STR = (Number of surfaces * Sectors per track * 512) / ( 2 * Number of
surfaces * Latency + (Number of surfaces - 1) * Head Switch Time +
Cylinder Switch Time)

- Remember that each disc in your HDD has two surfaces. The top and the
bottom.
- It is usually near to impossible to find the exact number of sectors
per track because the manufacturers don't like to release that
information Estimate between 300 and 500 for a modern HDD.
- Heads switch time and Cylinder Switch time can be estimated at around
2ms each.

The rest should be available to you in the technical specifications page
of your HDD (probably on the manufacturers website).

-- 
Patrick Shirkey - Boost Hardware Ltd.
http://www.boosthardware.com - For the discerning hardware connoisseur.
http://www.boosthardware.com/LAU/Linux_Audio_Users_Guide/
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