Digital I/O & Wordlength 189

D24—Owner’s Manual
Digital I/O & Wordlength

When a high-resolution digital audio signal is transferred to a lower-resolution system,

care must be taken to ensure that the digital audio samples are truncated correctly.

When a 24-bit signal is transferred directly to a 16-bit DAT recorder, for example,

low-level detail is lost when the eight least-significant bits (LSB) of each audio sample

are discarded, resulting in distortion and a grainy sound. The greater the truncation,

the greater the distortion.

To optimize the truncation process, a technique known as digital dither is applied

before feeding the signal to the lower-resolution system. Digital dither works by com-

paring the output of a special pseudorandom number sequence generator with the low-

est data bit of the shortened audio sample and the unwanted bits. The lowest bit is then

rounded up or down prior to D/A conversion.

Digital dither endows the digital signal with a noise signal (hiss) close to the minimum

theoretical limit, which is around –96 dB for a 16-bit signal, although this is considered

to be less offensive than the distortion caused by truncating without dither. To mini-

mize the noise, some digital audio processors use noise-shaping techniques to shift it

into areas of the audio spectrum where the ear is less sensitive. Unless you have a spe-

cific reason for not using it, digital dither should be used when transferring high-reso-

lution signals to lower-resolution systems.

In the following system, a 24-bit recording resolution is used for multitrack recording

on the D24, and a 16-bit resolution for stereo recording on the DAT. The digital dither

function on the digital mixing console is set accordingly.

Recording resolution: 24-bit
Recording resolution: 16-bit
000000 00
YAMAHAD24
OVER
READY
0
2
6
1012
1420
1830
2042
2660
–dB
OVERABS H
LOCK
TIME DISPLAY
CAPTUREABS/RELREMAIN
VARI
SPEEDUTILITY SETUP
PROJECT
SELECT
LOC MEM
RECALL
LOC MEM
STORE
LOCATEENTERCANCEL
0/-
1
4
7
8
R
7654321
RECORD
READY
SOLO/
SELECT
MONITOR SELECT
PEAK
HOLD AUTO
INPUTALL
INPUT FORMATCHASE
L
2
5
8
3
6
9
UNDO/
REDO
EDIT
JOG ON
JOG/DATASHUTTLE/
CURSOR
PROJECT SEARCH
RTN TO
ZERO
REW FF STOP PLAY REC
ROLL
BACK
LAST REC
SET
DIGITAL MULTITRACK RECORDER
REPEATREHE
PHONES
LEVEL
010
ON
POWER
OFF
PHONES
BA
B
A
AUTO
PUNCH
IN OUT
INT24 48K
MASTER
WC
BIT FS
TC
MSF
READY
0
2
6
12
20
30
42
60
–dB
V. TRACK
SELECT
mYGDAI YGDAI
(SLOT 1)
AES/EBU (24-bit)
S/PDIF (16-bit)
D24
DAT
00.00.00.00
DIGITAL STEREO
COAXIAL OUT
Digital input
DAT

02R

Slot Output Select
SLOT 1: Bus 1–8
Dither setup
BUS 1–8: 24-bit
Stereo out: 16-bit