Right at the moment the original Amiga A1000 was being introduced, Commodore was already advertising its ability to use a Genlock peripheral, and promising that Commodore's own Amiga 1300 Genlock would be "available soon". This must have been something Commodore saw as a unique technical differentiator for the Amiga vs. contemporary personal computers. And it does seem that Commodore was correct, since there would be RocGen, SuperGen, G-Lock, etc. third-party Amiga genlocks to follow, and the Amiga would find widespread use with these in video production.

I'm curious to know what specifically made the Amiga hardware well-suited to marrying it up with a Genlock peripheral, other than it just having strong bitmap graphics support. In other words, what did Amiga hardware have, which other computers lacked, for this rather specific hardware use-case?

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    Note Amiga gfx wasn't only 'bitmap', it was also strongly planar. That means, instead of one pixel being represented by one byte or a part of a byte or a sequence of bytes, as common in PC, its value was composited from multiple 'planes' - think several overlaid monochrome / 1 bit per pixel images. This system very naturally yields itself towards having a 'greenscreen layer' independent from the rest of graphics.
    – SF.
    Commented Nov 19, 2021 at 13:42
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    @SF Good point. That does seem like it might be relevant for resulting overlay effects.
    – Brian H
    Commented Nov 19, 2021 at 14:38
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    This link looks like useful reading for anyone interested in this question textfiles.com/computers/genlock.txt
    – knol
    Commented Nov 19, 2021 at 18:38
  • @knol Great find! Appears to be from CBM and based on original A1300 genlock. And confirms role of software controlled pixel switch.
    – Brian H
    Commented Nov 19, 2021 at 19:07
  • Genlock capability was also possible for the BBC Micro. Not very surprising, really! Commented Nov 19, 2021 at 22:25

2 Answers 2


Typically a similar computer system would internally generate all system and video clocks it needs to operate from a single clock source, like a crystal oscillator on a motherboard.

Thus the system would only be able to free-run asynchronously and without any possibility to be adjusted to run synchronously with another system.

The Amiga simply added two features to enable genlocking. It is possible to (1) use an external clock source as the system clock and to (2) switch the video chip horizontal and vertical sync pins to inputs so they can come from external source instead of internal system clock based sync generator.

Selecting an external clock source is made very easy. The video connector has one input pin (XCLK) for feeding in the external system clock signal, and another input pin (XCLKSEL) to select if the internal crystal oscillator clock or the externally fed clock will be used as the system clock, default selection being the internal crystal.

The external clock needs to have approximately the same frequency than the internal clock (e.g. 28.636363 MHz for NTSC Amiga), which can be adjusted up or down within some small tolerance (e.g. 1% to 2%), to allow the Amiga video horizontal rate to exactly match with the external video horizontal rate.

This by itself is enough to make the system to run at the same rate than an external system.

If the external system would simply adjust the external clock so that Amiga output horizontal sync matches with the horizontal sync of the external system, it could take a long time for the hsync phases to get aligned as the system clock needs to be tuned only very slowly.

It would take even longer to adjust Amiga vertical sync output to align with the vertical sync of the external video, assuming that it is even possible to align them, as the external video might have different parameters than the Amiga internal video sync generator. The external video could have some tolerance in the amount of horizontal lines or simply be interlaced or progressive.

That is why the Amiga video chip has a software controllable bit (ERSY in BPLCON0 register) to switch both the HSYNC and VSYNC pins to inputs, so that a device on the video connector can drive the sync signals into video chip which can then generate video according to external sync signals.

This way the video chip can be kept locked well to external video signals.

The actual magic of genlocking happens externally to the Amiga, in the genlock hardware connected to the video port. A genlock would have a sync extractor to produce the required seprarate HSYNC and VSYNC from the incoming video signal to video chip, and a PLL to multiply the HSYNC frequency by 1820 (NTSC) to produce a system clock that is aligned to have the exact same HSYNC to system clock relationship as the internal sync generator would have.

Now that the Amiga has been gotten genlocked with incoming video signal, and so it is in sync with all other devices for example in a TV studio, what happens next is up to any the external hardware connected to the Amiga. The Amiga itself will keep transmitting video as usual, and does not care how the video is used.

So in a TV studio setting, it can be used just like any other video source, mixing it with TV camera signals for example, or do any other processing - it depends on external equipment.

However, a typical Amiga genlock device simply allows to overlay Amiga generated graphics on top of existing video signal coming from some source, like VCR or camera.

This is possible, because the Amiga video chip outputs a signal named PIXLSW to the video connector for the genlock device to use. The signal simply indicates if the current pixel is the background color or something else. It means that the video output on the genlock device can be switched between original input video signal, or have it replaced by Amiga generated video signal. Background color will switch to original video, and any non-background pixels will replace original video. So basically the PIXLSW signal can be used as a mask to know what to pass through or replace.

  • Good explanation on how the two videos get synced up. But that (seems to me) is prerequisite only - how do the two signals actual mixing occur to produce the overlay effect?
    – Brian H
    Commented Nov 19, 2021 at 14:35
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    @BrianH Once you have the sync, you can just switch back and forth between passthrough and internally-generated video at pixel rate.
    – hobbs
    Commented Nov 19, 2021 at 15:25
  • @BrianH Oh good point - I can add something about that to the answer, but it really has nothing to do with genlocking ability of the Amiga itself, it has more to do with the genlock device, or a video mixer. It's really the same thing as with TV studio with multiple cameras - all cameras have to be locked on to same clock reference, so the video mixer can switch or mix or do effects with the different sources, as they are all in same horizontal and vertical sync.
    – Justme
    Commented Nov 19, 2021 at 15:39
  • @Justme I was imagining Amiga videos /PIXELSW signal on Pin 14 is also relevant, particularly if it allows interpolation of planar data.
    – Brian H
    Commented Nov 19, 2021 at 15:42
  • @hobbs. No doubt, but how?
    – Brian H
    Commented Nov 19, 2021 at 15:43

In other words, what did Amiga hardware have, which other computers lacked, for this rather specific hardware use-case?

It was the ability to let its video timing be synchronized from an external source - here the A1300 Genlock. So video generated by the Amiga follows the same timing and phase as the external source, enabling both to be mixed into a new signal.

Genlock isn't a big deal per se.

It only means that a video device can be synchronized to an external source, as the name already implies: GENeratorLOCKed.

Basically every TV set is a genlock device, as it locks its display timing to an external source, the TV signal.

In case of the Amiga, the process is essentially the same. A Genlock device, like the A1300, takes an external video signal and recreates synchronisation signals (picture start, line start) and feeds them towards the Amiga, where the AGNUS chip would use them to synchronize its output instead of its own signals, so the output is in step with whatever the genlock device sees from the external source.

[At that point the Amiga adds a speciality, as all its timing is related to the video timing (like with other early home computers). Thus the Genlock must also deliver a 20 MHz base clock. What does make this rather complex is that during synchronisation no 'beat' may be lost, i.e. both clocks must be brought 'slowly' into phase.]

It is this ability to sync (with a bit of help) to an external source, that simplifies the task of mixing two signals.

Videomixing is Neither

In fact, Cromemco's Super Dazzler already had the ability to genlock and mix its picture with an external source.

In theory the TI 99/4 could have done so as well, as its 9918 can incorporate an external video signal when synchronized and mix it with the internal graphics. There's a special colour 'transparent' to allow this. I do think the feature has been used by at least one (later) MSX system from Sony. But that's a different story.

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    output instead of his own signals - but, I thought all the amiga chips were female? It's an Amiga rather than an Amigo, after all ;)
    – Caius Jard
    Commented Nov 19, 2021 at 10:24
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    Angus is a male name, true, but the Amiga chip was called Agnus, and pronounced like the female name Agnes. The OCS also featured a chip referred to as Denise. They were replaced by Alice and Lisa, Paula was sound.. I forgot the others..
    – Caius Jard
    Commented Nov 19, 2021 at 13:39
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    Agnus is just a short for "Address GeNerator UnitS". So originally it had nothing to do with lambs. At least not until someone confused Agnus variant of the Greek name Agnes (meaning pure, holy) with lambs.
    – Justme
    Commented Nov 19, 2021 at 16:07
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    You all are forgetting poor Gary.
    – knol
    Commented Nov 19, 2021 at 18:37
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    @Justme: Or the Latin Agnus Dei "Lamb of God".
    – supercat
    Commented Nov 19, 2021 at 19:14

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