Originally Posted by GGA
I probably missed it but I have not been able to find defintions of ISD
, ISD termination,
and ISD gap
. Could someone please point me to a source for these definitions?
the Inter Signal Delay (ISD) gap is a length in time after the original signal reaches the listening position that is essentially anechoic (no reflections or specular energy is allowed to arrive within this window in time) - you are ONLY hearing the direct signal from the speakers and no other reflections from any boundaries within the room.
if you were in an anechoic room, your ISD gap would be infinity, as you'll never get a reflection off a boundary (as all reflections are fully attenuated - just as if you were outdoors or in an infinitely large room).
in small rooms where the boundaries are relatively close, reflections off the sidewalls and ceiling will take a slightly longer path than the direct signal, which means these reflections will arrive slightly longer in time and combine with the original signal at the listening position.
if a specular reflection arrives within the ISD gap, then it is considered to be an 'early' reflection. but it is important to note that not all first-order-reflections are early-reflections. in a large room, the reflection path is so great that the reflection arrives at the listening position outside of this gap - hence it is not necessary an 'early reflection'.
for example - a 20ms ISD means that once the original (direct) signal from the speaker reaches the listening position (straight line, hence the shortest path), there is a 20ms anechoic gap where no other specular energy impedes the listening position. this allows the brain to 'digest' the original signal in totality before also hearing the reflections within the room.
any early-reflections arriving at the listening position within 15ms or so of the original signal will cause time-smearing and issues related to clarity. any early reflections arriving in this window need to be identified and attenuated (either by redirection, absorption, etc)...if that is indeed the room model you are emulating and the specular response you are looking to achieve.
by attenuating these early reflections (or delaying the time in which they reach the listening position AFTER the original signal), you're also essentially tricking the brain into thinking you're in a larger room.
the termination of this gap is the first significant burst of energy (specular or diffused) that arrives after the anechoic ISD gap, hence "terminating" the gap.
there are psycho acoustic effects that relate to this termination (Haas effect). if you read some of SAC's commentary that i quoted on the previous page, this will be better explained.
it was born out of control room models (LEDE/RFZ) - which each have very specific criteria for how the specular energy arrives and decays at the listening position - but these room models are directly applicable to critical listening rooms (where the room needs to be made as neutral as possible, if that is the design requirement).
with regards to recording (which is not really relevant here, but it does add insight) ... the ITD (Initial Time Delay) is generally used for the length in time in the recording room, and the ISD (Initial Signal Delay) is generally referred to the gap within the control (mixing/listening) room.
so, if music was recorded in a large space where the first reflection arrived 30ms after the original signal ... and you were to play this back in your control room that is much smaller in space (eg first reflection arrives at 15ms) ... this means that your control/listening room is masking the recording., and instead of 'hearing' the room the music was recorded in, you're hearing your listening room mask its sound on top of the recording.
so, the general criteria is to have the ISD of the listening/control room be 2-5ms later than that of the recording room ... so you can hear the room in the recording in totality before your listening room has a chance to mask its sound on top. think about listening to a recording from a large concert hall, and playing it back (reproducing it) in your bathroom. you are going to 'hear' the small acoustical space via the early reflections, versus 'hearing' the concert hall ambiance. if you close your eyes, you'll be aware you're in a very small room vs being transported into 'being' in the room of the recording.
go into your bathroom and close your eyes and begin talking. when you talk, the reflections off the boundaries arrive back at your ear very quickly because of how close the boundaries are, and your brain processes this and you can 'hear' or 'feel' how small the room is. if you did this in a large room (movie theater, warehouse,etc) the reflections would take a much longer time to reach your ears (possibly turning the reflection into a distinct 'echo'), and you could 'feel' how large of a room you're in, even if you were blindfolded.