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Lawrence D'Oliveiro writes:
> I've heard some mention of [Acorn] Replay. One comment I heard was that it
> could only handle a fixed playback rate.
> 
> Can you tell us some more about Replay?

Here's a recent posting.

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Newsgroups: comp.sys.acorn.advocacy
Subject: Replay FACTS
Message-ID: <19883@acorn.co.uk>
From: RWilson@acorn.co.uk
Date: 30 Oct 92 10:17:21 GMT
Sender: rwilson@acorn.co.uk
Lines: 148

Dear Bob, (and other comp.sys.acorn.advocacy peeps)

You seem to have a real problem with Replay not shared by the
other users of it. You also seem to have some breathtaking
ignorance of its capabilities. If you have the Acorn Replay
CD 'VideoClip Collection One' please read the ProgIf and
AE7Doc files inside the !ARMovie resource directory.

> 1.  Quicktime has NO LIMIT upon the image size.  However, practically
>     it isn't wise to go much about 320x256 on anything other than a
>     top end Quadra.

Neither Replay, QuickTime nor MicroSoft's forthcoming system have any
technical limit on framesize. However you don't want to make large
movies in any of them since they become a sequence of stills... The
largest movie I have made in Replay was 320x256 (45 pixel per inch)
at 12.5 fps: i.e. it plays at full screen without using the 'Play Big'
option of Replay. It is very impressive, but uses too much bandwidth
to come off a 150KByte per second CD ROM drive: unlike Mac systems,
many Archimedes people don't have enough hard disc space to copy
movies to them... (the 42 second movie is 10724336 bytes!)
Replay can play a 12.5 fps on a 2MByte Archimedes (without going
mad reconfiguring it, either!).

> 2.  Quicktime has full support for stereo sound (16 bit samples)
>     although I admit I've never seen a movie which uses it.

Replay currently supports the following sound formats:
  8 bit exponential mono and stereo
  8 bit linear signed mono and stereo
  8 bit linear unsigned mono and stereo
 16 bit linear signed mono and stereo
  4 bit linear ADPCM (represents 16 bit linear) mono and stereo
The Replay 'VideoClip Collection One' has 8 and 16 bit stereo samples
of sound only Replay movies. 16 (and 4 bit ADPCM) sound is run time
converted to VIDC1 format on current machines. Replay also supports
movies with multiple sound tracks: a particular track is nominated
and the movie is played with that soundtrack.

> 3.  Quicktime replays a 256x192 image on a Quadra at 25fps provided
>     the media is fast enough (ie a fast CD ROM or a Winchester).
>     On lesser Macs such as the LC2 this drops to between 10 and 15 fps.

Replay plays from standard speed CD ROMs: on ARM2 machines at 12.5fps
and on ARM3 25 fps. Of course your CD ROM must work at this speed: many
people have poorly connected SCSI busses whose CD ROM drives are not
able to run at 150KBytes per second, which has caused a problem when
they first run Replay (though even these drives, retrying all the time,
can run at 12.5 fps...). You also MUST have CDFS 2.10 - earlier ones have
awe inspiring bugs! Of course, Replay movies can come from any fast enough
filing system: I have run them from hard disc (trivial), magneto-optic
disc (fairly easy: MO is 1/2 the speed of hard disc), CD-ROM (lowly Chinon
and nice Sony), NFS on Ethernet and AUN on Ethernet.

> 4.  Quicktime uses 16bpp graphics and quantizes on the fly (as does
>     Replay).  If you have a 16bpp graphics card it'll show it in
>     16bpp.  Replay can also do this with a PCATS Graphics Enhancer.

True. Ish. Replay uses 15bpp. Pixels can be YUV or RGB. In theory
Replay's decompressor addresses any of the 16bpp graphics cards around,
in practise I've only got the PCATS so it might not work on the newer
ones. Replay does realtime ordered dither to 8bpp (or less) screen modes.

> 5.  Quicktime MULTITASKS and uses LEGAL screen access.  ie.  a
>     quicktime window can be partly masked by other windows etc.
>     Replay, on the other hand, doesn't multitask and writes
>     directly to screen.

Replay writes directly to the screen. However it does so using all the
legal access methods, so works with arbitary modes and graphics enhancers.
Replay doesn't multitask: in the cooperative environment this would lead
to dropping the refresh rate... Replay does have a sound only multitasking
player.

> 6.  Quicktime movies can be played BACKWARDS (not just key frame
>     jumps, but genuine reverse play including the audio).

Replay can only be played forwards. Thus it can use certain compression
tricks... Of course, with sufficient store, combined use of the key
frames would allow a movie to be played backwards - but what's the
point?

> 7.  If Quicktime can't refresh fast enough it does NOT break
>     the audio.  The audio always (almost always..!) plays
>     unbroken, and the video jumps to catch it up.

Similar to Replay: audio continuity is only broken if the underlying
drive is not fast enough. (Quicktime can load the entire soundtrack
first whereas Replay loads it in blocks dynamically: hence, if the
source is not fast enough GET IT FIXED!).

> 8.  Quicktime can plot into irregularly shaped windows.  For example
>     one example movie is of a guy talking, and the window is the
>     exact shape of his head.  All this, and it still multitasks
>     (ie you can drag the window around whilst replaying).

Replay can't do this. However, in the latest version of the decompressor,
the window can be programmed to follow an arbitary trajectory while it
is decompressing. All still at 25fps displayed frames...

> 9.  Quicktime has hardware accelerator options.  New Macs are
>     likely to have this as standard, and cards for older Macs
>     aren't exorbitant.

Replay and QuickTIme both support arbitary compression systems: for the
Mac there are enough people willing to spend $2500 on a hardware board
that people make them. For the Archimedes only Irlam Instruments have
so far shown interest in hardware compression/decompression commercially:
for the ESPRIT project MULTIWORKS, a moving JPEG hardware board supporting
Replay has been built.

> 10. I have desrcribed the OLD version of Quicktime.  Quicktime 2
>     should be released very soon - I have no idea of the new
>     spec however.

QuickTime 1.5 - the just released version - is virtually identical to the
above with some bug fixes and the inclusion of a video compressor/
decompressor purchased by Apple from SuperMac technology: this new
decompressor is about 2x faster than the old one, but the associated
compressor can take as much as 45 seconds to compress a frame...


Replay news: Irlam Instruments are preparing a low cost capture card:
pour video directly onto your hard disc: all machines should manage
12.5fps, faster hard discs should manage 25fps. Only provides 1/4
screen (160x128 15bpp YUV) capture. Price tba, but Acorn User claims
250 pounds.

Replay now supports movies with an arbitary number of soundtracks
(see above).

Replay now supports arbitary trajectoriess (see above).

A patent for Replay's base compression and decompression technology is
being applied for: if its granted you can read it, if its not granted,
then I'll probably end up posting the technical info... Acorn are quite
happy to consider requests for licenses. (The A4's greyscale dithering
is already patented in the UK: ditto for requests).