Cedar DH-1 De-Hisster User manual

Professional Hardware Systems
DH-1 De-Hisser
Digital Audio Restoration System
SERIES 2
OWNER'S MANUAL
© 1994 CEDAR Audio Ltd. Written by Gordon Reid
DH- 1: Rev.02 Ver. 1.06
Page - 1
July 12, 1996

TABLE OF CONTENTS
TABLE OF CONTENTS.............................................................................................2
INTRODUCTION ........................................................................................................3
THE
BACKGROUND TO CEDAR NOISE REMOVAL.....................................4
SAFETY INSTRUCTIONS .................................... ... ..................................................9
SET
UP ........................................................................................................................... 11
Unpacking and Inspection............................................................................. 11
Installation site.................................................................................................. 11
Rack Mounting ............................................ .. ................................................... 11
Free Standing use ............................................................................................. 11
CONNECTIONS ..........................................................................................................12
Before
Connection ....................................... ... .................................................12
Power Connections ............................. .. ....................................... ... .................12
Signal
Lead Connections ................................................................................12
Other Connections ................................... .. .......... .. ..........................................14
SAMPLE INSTALLATION IDEAS.........................................................................15
A GUIDE
TO RESTORATION PROCESSING .. ............................... .. ..................16
FRONT
PANEL INDICATORS AND CONTROLS............................................. 18
QUICK TOUR...................................................... ... ............ ... .......................................20
WARMSTART
AND COLDSTART....................................................................... .21
OPERATING THE CEDAR DH-1............... .. ....... ... .................................................22
Dedica ted Con troIs ................. .... .... .. ..............................................................22
PAGES ...............................................................................................................24
Con trol Page ......................................................................................... 25
Input/Output Control Page .............................................................. 28
Remote Con trol ....................................................................................32
Status Page .......... .. .... .... .............. .. ........................................................33
Status Indicators...................................................................................33
TUTORIAL.....................
... ..... .. ...................... .. .............................................................35
THE TUTORIAL TAPE..............................................................................................38
REMOTE
CONTROL PROTOCOLS....... .. . ... ..........................................................39
RS232...................................................................................................................39
MIDI..............................................
.. .................................................................... 41
SELF TEST MODE ..................... ... .... ... .... .......... ... ..... .. ...............................................42
DH-1: Rev.02 Ver.1.06
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July 12,1996

INTRODUCTION
Thank you for purchasing the CEDAR DH-1 De-Hisser Module. This is the world's
most advanced dedicated single-ended noise removal unit, and offers processing
power and performance that could only previously be obtained using digital signal
processors (DSPs) installed
in desk-top (or larger) computer systems such as the
CEDAR Production System. The De-Hisser is designed for professional use,
although it will work perfectly well
in a domestic environment, and its features
include the following:
Revolutionary noise removal algorithms
• No need for a "Spectral Fingerprint"
• The latest 'SERIES-2' CEDAR hardware
Digital Audio interfaces conforming to the AES/EBU and SP-DIF standards
24-bit input and output resolution when using AES/EBU interfaces
• Three sample rates supported
on digital inputs: 32kHz, 44.1 kHz and 48kHz
• Two sample rates supported
on analogue inputs: 44.1 kHz and 48kHz
• Balanced analogue inputs and outputs for connection to professional
analogue equipment
• ADC and DAC converters using the latest 64x over-sampling
~- I. (Delta-
Sigma) technology
• >103dB dynamic range AID and >93dB dynamic range D/A
Mountable in a 19" EIA rack
• Remote control via MIDI and RS232 interfaces
• SMPTE/EBU timecode capabilities via optional upgrade
• Input and output LED bar-graph
VU meters
Twin 40-bit floating point DSP processors delivering 50MFIops to handle the
most complex audio processing requirements
• High levels of artificial intelligence designed into the DH-1 program
algorithms making
it extremely simple to use
DH-1: Rev . 02 Ver.1 . 06
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July 12. 1996

THE BACKGROUND TO CEDAR NOISE REMOVAL
Cheap digital audio (i.e. CD) has made discerning listeners quite intolerant of the
noises and distortions present
in analogue audio signals. After all, in a perfect
digital world there are
no clicks, crackle, pops, buzzes or hums, and no hiss - so it's
a shame that we live
in a far from perfect world. Even today, the vast majority of
mixing desks still have all-analogue signal paths, so most DOD-classified CDs are
still mastered through numerous analogue stages. And 'vintage' (i.e. pre-1982)
recordings are
by definition re-mastered from analogue master tapes which
inevitably suffer from at least one of the degradations listed above. So recording
engineers are turning more and more
to the technologies available for reducing
any noise added in the signal path, or for removing it from the final recording.
You can
rid yourself of any broadband noise you care to mention ... white noise,
tape hiss, microphone noise, rumbles
... you name it, you can eliminate it. Totally,
and without any fuss or expensive equipment. How? Simple
... by turning your
master volume control to zero. OK,
so this method also has a rather drastic effect on
the signal content of your recording - it completely removes it -but what do you
mean, you want to get rid of all the noise but keep the genuine signal absolutely
untouched?
Before proceeding any further, perhaps it would
be best to describe what we mean
by the term 'broadband' noise. Such noise is, by definition, a random effect which
adds (or subtracts) a random amplitude at all times to (or from) all frequencies
within the audio spectrum. Thus, the term
is used inappropriately to describe
artefacts such as intermittent electrical clicks or microphone 'grounding'. These
problems produce clearly identifiable events of limited duration, and may be
corrected
by the CEDAR DC-1 De-Clicker and CR-1 De-Crackler using quite
different methods to those described below. On the other hand, broadband noise
is
constantly present (to a greater or lesser degree) in every signal. It is often most
intrusive at high frequencies, where the masking effect of loud sounds is least
present,
so the term 'hiss' is often used to describe all forms of broadband noise.
Firstly, let's dispel any illusions regarding the Dolby
B, Dolby C, and dbx noise
reduction systems. These are dual-ended processes designed to minimise the
accumulation of any extra noise added by the limitations of analogue recording
tape. (Dual-ended processes are commonly called encode/decode systems
because the recording process 'encodes', and the playback process 'decodes', the
signal.) Neither the Dolby processes nor dbx enable you to remove noise from
within a signal that already contains
it -they simply stop you adding too much more
when you commit that signal to tape and then play
it back again. Perversely then,
both Dolby and dbx help your tape deck to accurately record, and then faithfully
reproduce, any noise contained
in the original signal. So, what you need is a
'single-ended' process that can remove noise from your signals prior to committing
them
to tape, or at the very least, can improve the signal to noise (S/N) ratio without
affecting the signal adversely. Which brings us neatly back
to the volume control. ..
stunningly effective at removing noise, it does nothing to improve the SIN ratio, and
has
an all-too-noticeable side-effect. No noise, No signal.
DH- 1: Rev.02 Ver .1.06
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July 12.1996

The first stage in our evolutionary tale of noise removal is the simple treble filter (or
'low-pass filter'). Less damaging than the volume control which removes the signal
a!together, the treble filter only removes a proportion of any signal present above a
g~ven frequency (known as the shelf frequency of the filter). Unfortunately, if, at the
given frequency, you reduce the amplitude of the noise content of your recording
by 6d8 (thus making the noise half as loud) you will also reduce the genuine signal
at this frequency by the same amount. This will be fine if your recording has little or
no high frequency content, but natural sounds and modern electronic instruments
have frequency responses up to and beyond the limits of human hearing.
Consequently, the treble filter will only be successful
in processing your antique
collection of '78's, and even then only
at a cost.
But this gives us a hint as to how a more effective single-ended noise reduction
system could
be designed: perhaps a device could be built which removes the high
frequencies when there
is no signal present, but leaves them untouched when the
noise is being masked by genuine high frequencies? Of course it can. It's a
Dynamic Filter (so called because the shelf frequency of the filter moves
dynamically up and down the frequency spectrum according to information
contained
in the signal). But such devices are limited: for one thing, they can only
remove the noise which exists above the highest frequency of the music present at
any given moment. Secondly, they are based
on filters with roll-offs typically of the
order -12d8/0ctave or -6dB/octave,
so they always allow some high frequencies
through, even when they think that they're removing
it. And thirdly, even though the
filters are designed to track the signal very quickly, they cannot respond
instantaneously, so they tend to round off fast transients such as snare drums and
samples (which have a habit of introducing high frequencies very rapidly into the
signal). And, because their raison d'etre is to reduce the signal bandwidth they also
tend to dull the genuine signal quite perceptibly. So
to summarise dynamic filters: if
you're not compromising the signal you may not
be removing as much noise as
you wish, and if you're removing all the noise you're probably damaging the
genuine signal.
Perhaps an alternative approach could give better results? Instead of altering the
frequency response of the signal to reduce the noise content, how about changing
the volume (amplitude response) of the signal
in some way? This isn't such a
strange idea. Consider: if noise is always present
in a signal then, if the total signal
amplitude drops down to the noise level, surely all the genuine musical signal has
disappeared? While there are many flaws
in this argument (largely to do with the
statistical nature of broadband noise)
it is, as a generalisation, nearly true. This
then suggests a device which will eliminate some of the noise: a Noise Gate. This
simple device detects when the signal drops below a certain level (a 'threshold' set
by the user) and then cuts off the signal entirely. It's just like turning the volume
control of your hi-fi to zero between tracks, and then turning
it back up at exactly the
moment the music starts again. There are many enhancements to the Gate idea,
such
as variable attack and release times, and hysteresis (all added to limit the
occurrence of damaging side-effects) but the principle always remains the same: if
the device decides that there is only noise present at its input,
it totally shuts off the
signal. There
is an adage that says that a multi-track studio can never have too
many noise gates because, while they are pretty useless at stereo mastering, they
are invaluable for shutting off the intrusive hisses, hums, and buzzes of temporarily
unused synths and guitar channels.
DH- 1: Rev.02 Ver. 1.06 Page - 5
July 12.1996