LMn Manual

Interface and feature reference
LM2n / LM6n Manual 21
PPM page
Fig. 15.: LM2n PPM page
The PPM page shows
a peak programme meter (PPM) for the sig-
nal currently being measured by the plug-in
a smaller version of the Radar display as
seen on the plug-ins Radar page and
the two Descriptors selected on the Settings
page using the Left Descriptor and Right De-
scriptor parameters.
The peak meters of LM2n and LM6n display
true-peak as specified in ITU-R BS.1770. True-
peak meters give a better indication of headroom
and risk of distortion in downstream equipment
such as sample rate converters, data reduction
systems and consumer electronics than digital
sample meters used e.g. in CD mastering. Note
that the standard level meters in most digital
workstations and mixers are only sample peak
(Final Cut, Avid, ProTools, Yamaha etc.) and
should only be used as a rough guideline of the
headroom.
Note that the meter scale is extended above 0
dBFS. Most consumer equipment distorts if you
see readings above 0. It’s not a problem to have
true-peak level going to -1 dBFS in production,
but legacy platforms (analog, NICAM etc.) and
some data-reduction codecs may distort unless
true-peak level is kept lower. With Dolby AC3
and with low bit rate codecs, -3 dBFS should
be considered the limit, while legacy platforms
requiring emphasis may need even further re-
striction. As described in EBU R128, it is recom-
mended to make full use of the headroom with
true-peaks going to -1dBFS in production, and
to only restrict peak level further during distribu-
tion/transmission.