There is a significant 'dead band' of around half a division where edge triggering doesn't seem to work (checked using sinusoidal and similar repetitive waveforms, exists at the the base of the signal for positive edge and vice versa),
Please can you define it more accurate. Do you have any documented and trusted data for this?
No, not using the cheap Feeltech Arb I have at home. I only checked up to 5 Vpp, too. The edge trigger drop-out may improve at higher V/div settings.
This is just general observations of behaviour. Finding the exact points would require better signals, and be a bit pointless as there is no specification.
Do you perhaps mean that trigger hysteresis is bad thing? There is trigger level hysteresis - and there need be. I believe that mostly peoples in most situations do not want zero hysteresis for edge trigger.
The algorithm used does have to ensure that a stable edge has been found. I would think that three data points would be sufficient to establish that, at a minimum.
Without knowing what the 'scope is expected to do it is difficult to differentiate a bug from intentional behaviour. However, the broader you make that 'dead band', the less useful the triggering is.
Also whope paper is good to read for better understanding digital trigger system. Albeit this Siglent model is lot of more simple machine but still it follow these basic principles.
It is the specific implementation that is the issue, not the principle. We don't know much about this. The specifications are quite sparse. (This isn't documented in the manner of a high-end Keysight or Tektronix 'scope.)
Referring to a white paper from another manufacturer doesn't tell you anything about Siglent's implementation.
Knowing how equipment really works is very extremely important for avoid misconceptions and false claims.
You have this backwards. Testing the equipment yields how it "really works". If we don't know how it is
supposed to work (insufficient documentation), then we cannot tell if it meets its specifications.
Previously in this thread was also other total mess where signal was noise and wondering how it show peak in trigger position. Jus due to lack of enough knowledge and experience about how these things work even in basic entry level things.
The behaviour I noted isn't correct. Just because some Rigol 'scopes also behave oddly in a similar manner doesn't make the result correct (and if I recall correctly, Siglent is the older company, so I would guess that the algorithms got 'borrowed' by Rigol) -- you have to consider what the ideal is, and if the result departs from that in a systematic and aberrant way.
I noted that the pseudo-'noise' from the Feeltech Arb I have is triggered on as expected -- all of the sinusoidal-based waveforms are centred as expected, with either rising or falling edge set. Real noise, such as amplified open input (setting the V/div to small values), produces some odd 'stacking' effects. In some circumstances, initial results will change by simply opening the trigger menu.
Given that triggering consistently and predictably on noise is not a practical concern (we use 'scopes to inspect signals, where defined trigger points are important), this may not matter for usage. It may indicate problems or limitations in the algorithms used, however, so noting such behaviour might be of some use to those writing the algorithms.
Then:
... and the line indicating the trigger level does indeed vanish under certain circumstances.
Can you define it with facts anbd details (data! instead of opinions). And the reasons why it should be different, taking into account all the relevant factors.
When you adjust the trigger level there is a horizontal line to indicate where that level is, along with the numeric indication. But that line will disappear under some circumstances. Also, it seems odd that if you change from DC to AC coupling the trigger level is still settable, but only the numeric indication is present (and persistent, it doesn't snap back to zero).
There are a few odd bugs of this type, and other people have already documented them fairly well here.
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