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Back Next Contents Digital Video for Multimedia: Considerations for Capture, Use and Delivery

Section 2: Digital video: issues and choices

Other considerations for reducing file size

There are other techniques for reducing file size. So far we have considered those that involve compression; size of window, frame rates and quality settings. Others include:

Tip: The considerations of window size, codec, frame rates, number of key frames and quality settings are not independent of each other and there are always tradeoffs and compromises to be made. Our advice is to experiment with combinations of the above at the start of any project involving digital video, having first established your audience, and the delivery machines available.

Delivery from CD-ROM

The table below shows the data transfer rates for different speed CD-ROM drives
CD-ROM driveData Transfer RateNotes
Single speed150 kB/s original based on CD audio
Dual speed300 kB/srelative to single speed
Quad speed600 kB/srelative to single speed

The above transfer rates, however, are only available if the central processing unit (CPU) is not occupied with other tasks. Playing video files from CDROM, that is, loading, decompressing and transferring to the video card, requires a lot of processing power and reduces the bandwidth available.

But by how much?

Unfortunately there are no exact answers. This is because of the number of factors :

When evaluating specifications of CDROM drives, the important parameters are the transfer rates at 60% and 40% CPU usage, not the 100% figures. This gives an indication on how the drive will perform when the CPU is busy doing something else like decompressing video.

How does this translate into actual figures?

CD-ROMData Transfer RateLikely Data Transfer Rates at 40 - 60% CPU Usage
Single speed150 kB/s120-150 kB/s
Dual speed300 kB/s210-230 kB/s
Quad speed600 kB/s300-350 kB/s

Note: within the case of single speed CD-ROM, 150 kB/s should be achievable by nearly all CPUs.

For double speed drives compression settings with data rates of 210-230kBs should be reasonable. Moving to a quad speed drive (600kBs-1 raw transfer rate), for the same specifications of computer as for a dual speed CD-ROM, the processor is now decompressing more data. Therefore 300-350kBs becomes the guideline range.

When compressing for delivery from CD-ROM and if not using MPEG-1, turn on the padding option, if available, in compression settings. Data is normally written to CD-ROM with a sector size of 2 kB. Padding adds dummy data to the frames to the nearest multiple of 2 kB thus ensuring that frames always start and stop at a sector boundary. Editing software provides default settings for each codec for key frame interval, compression quality, etc, and it is probably best to accept these until you are familiar enough with them to make changes.


When discussing digital video there is, almost always, the assumption that audio is included. Of course this is not always the case, there might not be any associated sound or it might not be important. Presented here are some of the issues involved with audio.

Any audio information is going to use some of the bandwidth available. The following table gives an indication of the data rates associated with some standard settings

Sampling frequency (kHz) MonoStereo
8 bit16 bit8 bit16 bit
11.02511 kB/s22 kB/s22 kB/s 44 kB/s
22.0522 kB/s44 kB/s44 kB/s 88 kB/s
44.144 kB/s88 kB/s 88 kB/s176 kB/s


As can be seen from the table the audio data alone can (eg stereo, 16 bit at 44.1 kHz) take more bandwidth than is available from a single speed CDROM.

What are the options available to reduce data rates?

This will, obviously, depend very much on the importance of the audio information to your video. Taking the highest data rate (176 kB/s) as a starting point consider:
  1. mono instead of stereo (reduce down to 88 kB/s)
  2. 22 not 44 kHz sampling (reduce down to 44 kB/s)
  3. reduce to 22 kB/s by choosing either:
  4. or both a) and b) to reduce to 11 kB/s

As usual there are other ways of tackling the problem. Again one of the most useful tools at our disposal is compression. Depending on the nature of the audio signal, there are a number of ways of compressing the data. These algorithms attempt to take account of such factors as the way we hear and the large amounts of silence that can occur in speech detected by there being no signal, etc. Fortunately there are industry standards/organisations, such as the Interactive Multimedia Association (IMA), that tackle such issues as:

For example, using the DVI® (Digital Video Interactive) algorithm, which consists of 4 bit ADPCM (Advanced Differential Pulse Coded Modulation) samples, we can achieve 44kHz, 16 bit (equivalent), stereo at a data rate of 44 kB/s (25% of the uncompressed rate).

Other tips and advice

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