Transcript Digital Audio Broadcasting

Advantages of Digital Broadcasting
• Better signal to noise ratio
• Reduced interference
• Possible single frequency networks
• Less multipath distortion
• Automatic tuning
• Auxiliary data services
DRM - Digital Radio Mondiale
• Developed by a group including broadcasters, transmitter
and receiver manufacturers, research labs, universities,
and government agencies.
• Designed to eventually replace AM in the present AM
bands (0.15 to 30 MHz).
• Transmitted bandwidth typically 9 kHz. Standard covers
4.5 to 20 kHz.
• Complete specifications documented with ETSI and ISO.
Consumer receivers just starting production.
• Different configurations for local and shortwave conditions.
Processing of Signals
• A/D conversion
• Audio processing
• Source coding - MPEG
AAC
• Forward error correction (FEC) coding
• OFDM (Orthogonal
frequency division
multiplex)
• Channel coding - time
+frequency interleave
• Digital modulation of
subcarriers
• Inverse Fast Fourier
Transform (IFFT)
• Interpolation
• RF frequency generation
and modulation in DDS
(direct digital synthesizer)
• Upconversion
• Power amplification
Audio Processing
• The minimum amount of control is automatic peak
limiting, that is, reducing the amplification whenever the
input level is so high that overload and clipping will be
produced in later steps.
• Audio levels in program material vary with the source
(live microphone, taped, satellite, CD’s etc and with the
skill and attention level of the operator. It is desirable to
have some automatic control of audio levels.
• When most listeners are located in noisy environments
such as automobiles, it is good to reduce the dynamic
range (ratio of soft to loud sounds) so that soft music or
speech will not be lost in the ambient noise.
Source Coding
• The output bit rates of the A/D converter are much greater
than can be transmitted in reasonable broadcast bandwidths.
(1536 kbits/second from the A/D converter for one channel.
• Fortunately it’s possible to reduce these bit rates drastically
with little or no perceptible loss of audio quality.
The coding technique for high quality audio signals uses
the properties of human sound perception by exploiting
the spectral and temporal masking effects of the ear.
• MPEG Advanced Audio Coding (AAC) perceptual coding
allows great bit rate reduction while maintaining excellent
audio quality. CD quality stereo requires only 96 to 128 k
bits/s. and good monaural AM 16 to 24 kb/s. It is specified
as the source coding method for DRM.
Error Correction Coding
• Bit errors due to noise, interference from other transmitters, and multipath propagation produce audible
clicks, squawks, and other noises at the receiver.
Forward Error Correction (FEC) coding adds calculated
bytes or bits to the signal to make it possible to detect
and in many cases correct erroneous bits at the receiver.
• Similar codes are used to make compact disks (CD’s)
and hard disk drives nearly error free in spite of
imperfections in the recording medium.
• Good error correction is a major consideration in the
design of these systems. FEC is most effective when
error bits are isolated or come in short bursts.
Error Correction Coding (2)
• Multipath propagation tends to make the
delayed signal interfere with later bits of the
desired signal. It can also result in a
delayed signal being 180 deg. out of phase
and canceling a desired signal. With a
multicarrier signal this normally affects one
or a few adjacent carriers at a given time.
The bits of the data stream are scattered
among the carriers so that errors on several
adjacent carriers become isolated errors in
the reconstituted bit stream.
Orthogonal Frequency Division
Multiplex
•Uses a relatively large number of subcarriers that fill the
allocated bandwidth with a low data rate per carrier.
•Used to minimize multipath problems.
•
DRM Mode A
- 24 carriers/kHz = 204 in 9 kHz
•
Mode B (SW) - 21 carriers/kHz = 182 in 9 kHz
•Modulation of each carrier is QAM 4, 16, or 64 states.
•Synchronization signals are distributed thru each frame
to give the receiver a phase and amplitude reference for
each carrier.
History of OFDM
• OFDM has been used in radio communication
for 50 years. The Collins Kineplex system in
the 1950’s put 24 300-baud teletype channels
on a single SSB transmitter. The mathematical
theory of the system probably dates from the
1930’s.
• Implementation of OFDM with analog circuits
was very complex. The 24 channel receiver
took up nearly all of a 6 foot rack.
Phase and Amplitude
Modulation of Subcarriers
• Various possibilities
• Simplest form is
BPSK (binary phase
shift keying)
• Reference phase = 0
degrees (0 Logical)
• Other state
= 180
degrees (1 Logical)
• 1 bit / Hz
Quadrature Amplitude Modulation
Quadrature Amplitude Modulation (2)
Inverse Fast Fourier Transform
• Converts desired frequency spectrum to inphase and quadrature waveforms
• Efficiently executed in DSP (digital signal
processor)
Interpolation (smoothing)
• Output of IFFT is a stepped waveform
• Must be smoothed to avoid producing
spurious sidebands
Power Amplification
•Typical peak-to-average power ratio for multi-carrier
systems is 10:1 (10 dB) with infrequent higher peaks.
•DRM requires high power for international broadcasting
(25 to 100 kW average power)
•A linear amplifier will have about 20 % efficiency for this
type of signal. For example, 125 kW dc in for 25 kW
average power out.
• A multicarrier signal can be amplified with 75 to 80%
efficiency by splitting it into two signals.
1. A constant amplitude RF signal with the phase variations
of the desired output signal (including phase reversals).
2. An amplitude (envelope) signal defining the
instantaneous amplitude of the desired output.
Power Amplification (2)
• The phase modulated signal can be amplified by the
class C intermediate and driver amplifiers of a typical
local AM or short wave transmitter.
• A class C anode modulated amplifier effectively
multiplies the phase modulated signal by its anode
voltage (the envelope signal), producing the desired
output.
• The time delay (phase shift) of the phase and
amplitude signals must be matched to a fraction of a
microsecond . The OFDM signal contains phase
reversals (points at which the amplitude goes rapidly
to 0 and then increases with reversed phase).
• Mathematical
analysis shows that
though the OFDM I/Q
signal is limited to its
designated bandwidth
with only a small
amount of out-ofband energy, the
phase and amplitude
signals both are
wideband. If they are
properly matched
their out-of-band
sidebands cancel.
•Spectra of signals for
amplitude-phase
amplification
Transmitter Requirements
• The fundamental requirement is that the
transmitter function as a linear amplifier and this
section shows how linear amplification can be
achieved with existing AM broadcast transmitter
designs.
• As has been described in the previous section, the
DRM signal from the OFDM modulator takes the
form of a group of equally spaced carriers, with
the digital information being modulated onto the
carriers in terms of phase and amplitude.
• This signal is termed an I/Q signal, as it is
complex, and contains In-phase and Quadrature
components.
• In raising the level of this signal to the power required
for broadcast transmission it is imperative that the
correct phase and amplitude relationship of the "I"
and "Q" components is maintained. In other words,
the DRM signal must not be distorted in the power
amplification process.
• If the signal is distorted, errors will be introduced and
the Bit Error Rate (BER) may fall to unacceptable
levels and the DRM signal be unusable and/or out-ofband radiation become excessive. To avoid distortion,
the power amplifier must therefore have a linear
transfer function such that the output signal is an
exact replica of the input but at a higher power level.
• A factor that has to be borne in mind for a DRM
amplifier, is the peak to mean ratio of the DRM
signal. The power level of a DRM signal is generally
stated in terms of its mean or average value;
however the instantaneous peak amplitude of the
combined carriers exceeds this value by a large
amount. A typical DRM signal has a peak to mean
ratio of 10dB, thus an amplifier producing a signal
having an average power of 10kW needs a peak
power capability of 100kW.
• In a practical amplifier, a point is reached where
gain compression occurs and the transfer
characteristic flattens out. The amplifier must be
operated on the linear portion of its characteristic.
• It is possible to construct a linear amplifier to provide
the power level required for broadcast transmission, but
its energy conversion efficiency is very poor, typically
20 – 30%; significant cooling is required and operating
costs will be high.
• Although some earlier low power transmitters used
linear amplifiers, high power AM transmitters invariably
use non-linear class C operation to achieve high
conversion efficiency. In a valve (electronic tube)
transmitter, the grid bias voltage is chosen such that the
valve conducts over a limited range of the RF cycle and
effectively delivers energy to the resonant anode circuit
as a series of pulses. The resulting RF power is coupled
from the anode circuit to the antenna.
• With the use of modern high power valve technology
and efficient cooling systems, very high output power
can be achieved for relatively low drive power with high
conversion efficiency.
• Solid-state modular MF/LF transmitters use a switching
technique to achieve high conversion efficiency,
typically between 70 and 80%. The output stage of
each power amplifier module uses MOSFET transistors
as switches arranged in an "H Bridge" arrangement. RF
power is taken from a transformer connected between
the mid-points of each arm. In operation, diagonally
opposite transistors are sequentially switched at carrier
frequency rate to produce alternate current reversals in
the output transformer primary. In this way high RF
power levels are generated at high conversion
efficiency.
• These non-linear amplifiers cannot be used directly
for DRM signals. However a modulated non-linear
amplifier can be driven with suitable RF and baseband signals derived from the original low level
complex I/Q signal, such that the component signals
combine in the modulated final amplifier to form a
high level replica of the original signal.
• The overall effect is that the modulated amplifier
functions as a linear amplifier even though the
amplifier itself continues to work in a non-linear
manner.
• Although a modular solid-state MF/LF transmitter
does not have a separate modulated amplifier as
such, the functionality is identical.
• The two signals derived from the basic DRM
I/Q signal are termed the RF Phase (RFP) and
Amplitude (A). The processing to derive these
signals is contained within the DRM Modulator
and generally the modulator will provide both
I/Q and A/RFP outputs.
• For this technique to work correctly there are a
number of requirements that must be satisfied
by the transmitter. Firstly, there must be a
direct (DC) connection between the modulator
and the final amplifier. Unfortunately this
means that the A/RFP technique cannot be
used with transmitters having Class B
transformer coupled modulators.
• For this reason the widely used Continental 418
A-F short wave transmitters cannot be used for
DRM without modification, but Continental can
supply a G upgrade using a pulse step modulator
which makes the transmitter DRM capable.
• Operation in a DRM simulcast mode does not
require DC coupling, but it still requires a greater
audio bandwidth than is possible with typical
modulation transformers.
• The Harris HF100 PDM modulator can be DC
coupled but it is necessary to raise the PDM
switching frequency to get the needed audio
bandwidth.
• Secondly, the bandwidth of the audio path in the
transmitter needs to be significantly greater than that
required for normal AM working. Typically, it should be
three to four times the bandwidth of the wanted DRM
signal.
• The sampling frequency of solid-state Pulse Step or
Pulse Duration Modulators (PDM/PSM) must be more
than twice this frequency limit to meet Nyquist criteria.
• Any bandwidth limiting filters in the audio path must
be removed and the modulator output filter will need
to be modified to achieve the required bandwidth. In
modifying the filter response it is important to ensure
that a substantially flat group delay characteristic is
maintained over the pass-band.
• In modifying filters there are two points to
bear in mind. Firstly, in some solid-state
modulator designs the modulator output is
slightly inductive and this will need to be
taken into account when designing the filter.
Secondly, the final amplifier valve provides
the load for the filter. It is important to
remember that although this is predominantly
resistive, the transmitter RF output circuit will
provide some shunt capacitance. Depending
on output circuit configuration, the shunt
capacitance existing at some tuning settings
may be significant and need to be taken into
account in the filter design.
• Current HF transmitter designs use a tetrode valve as
the modulated amplifier. In order to achieve linear
modulation to 100% with a tetrode, it is necessary to
apply modulation to the screen grid electrode. This is
usually done by applying a fraction of the modulating
signal to the screen grid, or by allowing the screen grid
to "self modulate", by including an inductance in the
screen grid circuit. The audio component in the screen
current develops an AC voltage across the inductor and
this AC voltage serves to modulate the screen.
• It is also necessary to decouple the screen grid, so that
it is at RF ground potential, for proper operation of the
amplifier.
• In some transmitter designs the value of decoupling
capacitance used may be sufficiently high to become
significant at the higher frequencies now required to
be handled in the amplitude path. This can degrade
the response at the upper end of the pass band. It
may therefore be necessary to reduce the value of the
screen decoupling capacitor, but this must be done
with great care in order not to disturb the RF
performance of the amplifier.
• The decoupling capacitor is often an integral part of
the final amplifier valve socket made in the form of an
annular ring of dielectric film, with electrodes
deposited on each side of the ring. One possible
technique is to add a second ring – this effectively
halves the decoupling capacitance.
• Even when all of the above points have been
taken into account, it may not always be possible
to directly achieve the required bandwidth and
group delay response from the transmitter. In this
situation some degree of pre-correction may be
needed. DRM Modulators now include some form
of pre-correction. In some implementations, the
pre-correction is user-adjustable and is set on
test to give the best response, in others, the precorrection is set at the factory and detailed
response measurements of the transmitter are
taken and provided to the DRM Modulator
supplier, who sets the pre-correction.
• For frequency agile HF transmitters, it will
generally be necessary to use some form of
dynamic phase equalisation in order to retain
the correct phase relationship between the A
and RFP signals as the transmitter switches
between the various broadcast bands.
Dynamic phase equalisation is generally
provided as an option with a DRM Modulator.
Transmitter Requirements
• The HC100 and other modern transmitters using pulsestep modulation can be modified to transmit OFDM signal in
conformance with the DRM specification. They typically
have 10 to 150 microseconds of delay in their amplitude
circuits but only a few microseconds in the RF amplifiers.
• (1) An audio server includes A/D conversion, audio
processing and MPEG source encoding to deliver a signal at
the required bitrate (An industrial PC).
• (2) An OFDM exciter implements Forward Error Correction,
Channel Coding, the IFFT and interpolation, in-phase (I)
and quadrature (Q) signal generation, derives Amplitude
and Phase signals from I and Q, compensates for time
delay of the amplitude signal, and sends phase and
operating frequency data to the Direct Digital Synthesizer
(one PCB or DSP and many lines of embedded software).
HC100 Changes
• (1) An Audio Controller with a higher switching rate to
produce the fast amplitude transitions of the OFDM
signal and control the screen supply. (One PCB)
• (2) A high voltage low pass filter with wider bandwidth
and less phase shift
• (3) A modulated Power Amplifier Screen supply.
Basically a simplified low power version of the pulsestep modulated anode (plate) supply.
• (4) Redesign of the fiber optic links and switching
modules in the modulator for more uniform turn-on and
turn-off times.
• This redesign is in process at the Engineering Center in
Elkhart