Interoperability of FM Composite Multiplex Signals in an IP Based STL

Interoperability of FM Composite Multiplex Signals in an IP Based STL
Interoperability of FM Composite
Multiplex Signals in an IP Based STL
April 23, 2017
NAB Show 2017
Featuring
GatesAir’s
Junius Kim
Hardware Engineer
Keyur Parikh
Director, Intraplex
Copyright © 2017 GatesAir, Inc. All rights reserved.
FM MPX Generation
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L+R
19 kHz pilot tone
L-R – 38 kHz subcarrier
Radio Data System (RDS) low bit rate (1187 bps) digital
data – 57 kHz subcarrier
FM MPX Frequency Spectrum
• Pilot tone decoded at
receiver
• Subsidiary Communication
Authorization – Low BW
audio
• 67 and 92 kHz Subcarriers
• FM MPX BW
• 53 kHz: L and R audio
• 60 kHz: plus RDS
• 75 kHz: plus one SCA
• 99 kHz: plus one SCA
RF Amplifier
RF Out
RF Amplifier
RF Out
Digital Modulator
FM Exciter
Digital
MPX over AES
Stereo Generator
Digital Signal Processing
Modulator
Analog
MPX Signal
Stereo Generator
Audio Processing
Audio Processing
FM MPX Generator
AES/EBU
• FM Audio Processing
• Frequency Limiting (< 15
kHz)
• Overshoot Limiting
• Pre-emphasis
• Stereo Generation
• FM Exciter
• FM MPX
• Analog MPX
• Digital MPX over
AES/EBU @ 192 ksps
Analog L/R
Audio
FM Processing Chain
Digital FM Exciter
MPX over AES3
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•
•
•
•
Introduced in 2013
AES3 or AES/EBU
– Two 32-bit sub-frames
– Sub-frame 32 bits = 24 bit sample word +
– Parity, metadata, and synchronization
192 kHz sampling
Left channel
192 kHz digital sampling = 80 kHz bandwidth due to
Nyquist
MPX over AES3
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•
•
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MPX over AES3 at 384 kHz
Support for the entire 99 kHz MPX bandwidth
Use left and right channel
Multiplex L and R in odd even sequence
Back compatible with MPX over AES @ 192 kHz
Energy from 96 to 99 kHz aliased to 93 to 96 kHz
Analog MPX
(0 to 100 kHz)
AES/EBU
192 kHz
Audio Codec
IP Network
MPX Codec
• STL Transport
– Baseband Audio
– Analog MPX
– Digital MPX
IP Network
Analog MPX
(0 to 100 kHz)
IP Network
Audio Codec
SCA 2
Audio Codec
SCA 1
MPX Codec
RDS
L/R Audio
(Analog or AES)
Audio Codec
STL Topologies
L/R Audio
(Analog or AES)
AES/EBU
192 kHz
RDS
SCA 1
SCA 2
Analog MPX Codec STL
Analog MPX
(0 to 100 kHz)
Antialiasing
LPF
ADC
Clock
Studio Site
PCM Data
to IP
Packets
IP Network
IP Packets
to PCM
Data
DAC
LPF
Clock
Transmitter Site
Analog MPX
(0 to 100 kHz)
Stereo Generator
A n a lo g M P X
M P X E n co d e r a t S tu d io S it e
( u p to 99 k H z)
S a m p lin g @ 1 3 2/ 1 6 2/ 1 92 / 2 16
k H z w ith 1 6 / 20 / 2 4- b it w o r d s iz e
A n a lo g M P X G e n e ra t o r
> 2 . 11 M b p s
RF O ut
A n a lo g M P X
Modulator
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•
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Audio Processing
•
Stereo Audio + RDS < 60 kHz
Stereo Audio + RDS + one SCA < 75
kHz
Stereo Audio + RDS + two SCA < 99
kHz
132, 162 or 216 ksps
6 dB per bit
24-bit word = 144 dB dynamic range
16-bit word = 96 dB dynamic range
132 kHz sampling, 16-bit word has
data rate of 2.11 Mbps
RF Amplifier
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•
RDS
E n c o d er
L and R Audio
Analog MPX over a STL
D a ta
A n a lo g F M E x c it er
( u p to 99 k H z)
IP
N e tw o r k
M P X D e co d e r a t T ra n s m itt e r S it e
S a m p lin g @ 1 3 2/ 1 6 2/ 1 92 / 2 16 k H z
w ith 1 6 /2 0 / 24 - b it w o r d s iz e
Digital MPX over a STL using
Transparent Transport
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End-to-end, bit-by-bit copy
Only transport the AES3 24-bit left
sample word
Regenerate parity, sync, metadata
at the far-end
One channel of 192 kHz, 24-bit has
a data rate of 4.6 Mbps
Digital MPX over a STL with
Reduced Bandwidth
• Reduce word size and use SRC to
reduce bandwidth
• MPX over AES at 192 ksps
supports 96 kHz bandwidth
• Stereo Audio and RDS < 60 kHz
• 132 kHz sampling, 16-bit word
has data rate of 2.11 Mbps
Bridging
MPX Bridging
M P X o ve r AE S
@ 192 kHz
4 .6 M b p s
RF Amplifier
RF Amplifier
Stereo Generator
D ig ita l M P X G e n e r a to r
RF O ut
A n a lo g F M E x c it e r
M P X o ve r AE S
@ 192 kHz
4 .6 M b p s
Modulator
A n a lo g M P X
( u p to 1 0 0 k H z )
Modulator
Stereo Generator
Audio Processing
A n a lo g M P X
( u p to 1 0 0 k H z )
A n a lo g M P X G e n e ra t o r
Audio Processing
•
L and R Audio
•
Bridge between analog and
digital domains for
interoperability
Interoperate between old and
new equipment
Dual domain input/output
provides “future proof”
solution
L and R Audio
•
T r a n s m itte r S ite
S tu d io S ite
RF O ut
D ig ita l F M E x c ite r
MPX Bridging – Digital
to Analog
•
Bridge between a newer digital FM
stereo generator and older FM
exciter
MPX Bridging – Analog
to Digital
•
Bridge between an older analog
MPX stereo generator and a new
digital FM exciter
MPX STL Bandwidth
6
5
IP Bandwidth, Mbps
• MPX is linear PCM – uncompressed
• MPX over AES/EBU is 192 ksps @ 24
bit sampling, one channel – 4.6 Mbps
• Analog MPX sampling is 132 to 216
ksps
• MPX IP transport uses RTP
• IP RTP/UDP header overhead is 40
bytes
• Tradeoff between delay and packing
efficiency
4
Series1
Series2
3
Series3
Series4
2
Series5
Series6
1
0
1
2
3
Sampling Rate, ksps
4
FM MPX over IP
Main Audio,
RDS,
SCA
FM
MPX
STL
IP Network
FM
MPX
Benefits of FM MPX over IP vs Audio over IP
• Enables baseband equipment (audio processor, stereo generator, RDS
generator) to be located at the studio side
• Reduces CapEx when distributing the same signal to multiple transmit sites
• Simplifies operation for FM SFN
• However – MPX requires higher STL capacity than audio only transport
• Audio is amiable to lossy compression – AAC, MPEG, opus, etc
•
Analog MPX Codec Requirements
Band from 0 to 53 kHz contains stereo audio
Left:
2L = (L+R) + (L-R)
Right:
2R = (L+R) – (L-R)
Gain flatness of 0.05 dB across 0 to 53 kHz for >50 dB stereo
separation
• Linear phase response
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Linear Phase
• Linear phase = constant group
delay
• Use FIR filters for linear phase
• Use over-sampling at ADC to
minimize effect non-linear phase
response analog filters
• Use interpolation at DAC to
minimize effect non-linear phase
response analog filter
Single Frequency Simulcasting
RF Amplifier
Modulator
MPX Signal
MPX Codec
IP Network
FM Exciter
RF Amplifier
Modulator
MPX Signal
FM Exciter
MPX Codec
MPX
Signal
MPX Codec
• RF single frequency simulcasting uses
multiple, geographically disperse RF
transmitters operating on the same
carrier frequency
• In simulcast, modulating signal
undergoes a precision delay process
• MPX advantage vs audio only transport
over STL
• In MPX all components are equally
delayed
GPS
IP Packet Loss
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Causes of IP packet loss: route flapping, transmission errors,
congestion
Unmanaged vs. managed network services
In audio - packet loss concealment methods: frequency
interpolation, replaying previous frame
In MPX – no standardized concealment methods
For MPX, use correction techniques for packet loss mitigation
Packet Loss Effects
• Audio compression algorithms keep
spectral information – fill in missing
data segment from previous data –
error concealment
• MPX codec method is PCM coding –
no spectral information is
computed
• MPX codec – no error concealment
IP Packet Loss
• Random vs. Burst Packet Loss
• Random Losses
• Uncorrelated
• Appear to be spread out
Random Loss
Burst Loss
RTP Forward Error Correction
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FEC packets are generated from a matrix of RTP data packets
Both RTP data and FEC packets are sent to the receiver
FEC attempts recovery of lost data packets at the receiver
Effectiveness of recovery depends on type of packet loss
FEC Matrix
FEC Correction for Random Loss
1.4
1.2
1
E
P 0.8
L
, 0.6
% 0.4
4x6
4x4
3x3
2x2
0.2
0
1
2
3
4
5
6
7
8
9
Network Loss, %
10
11
12
13
14
Single Network Packet Protection
Time Delay
Stream 2
Stream 1
100
101
101
100
102
FEC
103
104
Private/ISP
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For burst loss, packet level FEC with interleaving – adds delay
Or, add redundant streams in a group with programmable time delay. Very effective for burst packet
losses which are typically seen on public ISP connections
Time delay value based on network analytics
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Multiple Network Packet Protection
Time Delay
Stream 2
Stream 1
100
101
101
100
102
FEC
103
104
WAN 1
WAN 2
Stream 3
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•
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•
100
101
102
FEC
103
104
Network diversity
Grouped streams sent across diverse network paths
Scalable protection per network based on capacity
“Hitless” operation with packet and network losses
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Summary
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High bandwidth IP connections is an enabler for MPX transport
MPX STL - advantage of centralization at studio and simulcasting
Two methods of MPX – MPX over AES and analog MPX
Interoperability possible with a bridging device
For high quality MPX STL, IP packet loss must be mitigated
• FEC
• Redundant streaming
• Network diversity
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