Datasheet

HH
VS1053B OGG VORBIS ENCODER VSMPG
2.4.3 Building a Useful VU Meter
In an encoder application, if the recording level is too low, extraneous background noise
may be introduced to the sound. Conversely, if the recording level is so high that the
highest values cannot be represented numerically, signal clipping occurs, and this may
cause severe distortion to sound.
In a recording device, it is useful to have a VU meter that shows the signal level so that
both too low signal levels and clipping is avoided. This is very important so that the user
has a chance to either adjust the recording or input signal level.
0−6−12−20−30−40
−40 dB = 161
−30 dB = 512
−20 dB = 1625
−12 dB = 4096
−6 dB = 8192
0 dB = 16384
+5 dB = 29193
OVERLOAD = 29193
+5
OVERLOAD
Figure 1: Example colour VU meter.
0−6−12−20−30−40 +5
OVERLOAD
Figure 2: Example monochrome VU meter.
Figures 1 and 2 show example VU meters. The 0 dB reference point has been set to sig-
nal level 16384, which is one half of the maximum amplitude, leaving a 6 dB headroom
for the device.
The lowest signal level shown is a choice that can vary a lot depending on the appli-
cation. While -40 dB is a high-fidelity favourite, a VU meter will work just as well with a
lower limit of -20 dB. If display space is scarce, low limit could even be set to -12 dB.
Between -6 and +6 dB the VU meter precision should preferably be 1 dB, and at most
2 dB. Below -12 dB step size can be several decibels.
The important thing in a VU meter is to visually show the user that it is not recommended
to regularly use the highest 6 dB of the recording scale, and that an overload situation
(≥ +5 dB, or over linear value 29193) is an error condition. To help this the OVERLOAD
symbol should be kept lit or blinking for at least 2 seconds each time an overload situa-
tion occurs. Many of these cues are often ignored in digital VU meters, and partly as a
result of this even professional recordings are all too often made at recording levels that
destroy signal integrity.
Rev. 1.70c 2012-11-16
Page 14(39)