AMSTRAD cpc 6128 User Instruction page 143

If2 parameters are used, these are:
<hardware envelope> , <envelope period>
Parameter 1: <hardware envelope>
This parameter specifies the value to send to the envelope shape register of the
sound chip.
Parameter 2: <envelope period>
This parameter specifies the value to send to the envelope period registers of the
sound chip.
Knowledge of hardware is assumed when using hardware envelopes. Unless you
have such knowledge, it is suggested that you use a software envelope incorporating
a suitable <pause time> parameter.
General
Note that the total length of all the <pause time>s should not be greater than the
<duration> parameter in the SOU N D command, otherwise the sound will finish before
all the volume steps have been passed through. (In such a case, the remaining
contents of the volume envelope are discarded.)
Likewise, if the <duration> parameter in the SOU N D command is longer than the
total length of all the <pause time>s, the sound will continue after all of the volume
steps have been passed through, and will remain constant at the final level.
Up to 5 different <envelope section>s, (each made up of the above 2 or 3 parameters)
may be used in an EN V command.
The first step of a volume envelope is executed immediately.
Each time a given volume envelope is set, its previous value is lost.
Specifying an <envelope number> with no <envelope section>s cancels any previous
setting.
Further information concerning sound will be found in part 2 of the chapter entitled
'At your leisure .... '.
Associated keywords: ENT, SOU N D
Chapter 3 Page 24 Complete List of Keywords