Seagate ST15150N - Barracuda 4.3 GB Hard Drive Manuel d'utilisateur télécharger pdf (Page 8)

INTRODUCTION

If you are a Digital Audio Workstation (DAW) user and are tired of waiting for your analog tape

VTR to locate and follow up with your DAW, you would need to replace your video tape

machine with a random access digital video recorder/player that would respond instantly to your

locate commands and communicate efficiently with your DAW to allow you to finish your work

faster.

If you have an application that requires instant locate and playback from any time code position

you would need a random access digital video recorder/player.

The V1 is the first generation of our random access digital video equipment that uses magnetic

(hard drives) or magneto optical (MOD) drives as a recording medium.

To be able to record video on a hard disk it should be digitized which means that the analog

video information must be converted to a digital data stream. Every frame of NTSC (or PAL)

video contains 525 (or 625) lines that has 858 (or 864)pixels each. In a typical A/D conversion

every pixel is coded on 16 bits (2 Bytes), which yields a data stream of:

NTSC: 525*858*2=900900 bytes/frame or 29.97*900900=27 MB/s.

PAL: 625*864*2= 1080000 bytes/frame or 25*1080000=27 MB /s.

You see that in both cases the drive should be capable of handling a transfer rate of at least

27MB/s. This figure does not include any audio tracks. Since the transfer rate of various media

range between 1MB/s and 16MB/s, to record the video you would need to use RAID systems

(multiple drives chained together to achieve faster transfers) or compress the video data stream

by sacrificing picture quality.

Two compression techniques are becoming popular: Motion JPEG and MPEG.

Motion JPEG consists on compressing every field of video and save the data on the drive

MPEG consists on compressing only few fields/sec called reference fields and then recording the

difference between each new field and the reference fields. MPEG compression requires very

sophisticated techniques but yields a better transfer rate than JPEG for the same video quality.

Since the price of the media is going down day after day, the MJPEG solution is still the most

viable solution for a lot of applications.

The V1 uses a constant block size (CBS) Motion JPEG algorithm. With traditional JPEG

algorithms, depending on video complexity, the size of each JPEG field can vary thus requiring

maintaining a list to indicate the start of each field on the drive. In the CBS all fields have the

same maximum size. This is an overkill for non-complex pictures but it does not require

maintaining a list indicating the start of each field because they are all the same size.

Since not all lines and pixels are useful, the V1 only compresses the valid 480 lines and 720

pixels/line for NTSC (576 lines and 720 pixels/line for PAL) thus making the non-compressed

data stream 20.71 MB/s in NTSC (20.73 MB/s in PAL).

In addition to the video, and regardless of the compression ratio used, the V1 records 0, 2 or 4

tracks of uncompressed audio (sampled at 48Khz), one time code track and allocate space on

each drive for saving the setup information. Each audio sample is coded on 2 bytes (2*2*48000=

192 KB/s) and every field of time code is sampled on 80 bytes (29.97*2*80= 4.795 KB/s for 2

channels in NTSC and 25*2*6= 4 KB/s for 2 channels in PAL).

Hopefully, this introduction to digital video, has explained to the reader the principles of digital

video recording.

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Seagate ST15150N - Barracuda 4.3 GB Hard Drive Manuel d'utilisateur Page 8

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