Keyur Parikh is a solutions architect for GatesAir. This is one in a series of Q&As with industry professionals about their presentations at the upcoming NAB Show in Las Vegas.
Radio World: The session that you are participating in is called, “Distribution of Analog or Digital FM Composite Multiplex Signals Across IP Networks” (Sunday, April 17, 11:30–noon). IP technology has been the growth area for broadcast equipment makers in the last couple of years and there’s no indication it’s going to stop anytime soon. Sending an FM MPX along it would seem to be the ultimate goal. What could go wrong?
Keyur Parikh: Distribution of FM MPX over IP requires significantly higher network capacity, which needs to be handled by all the devices in the transport chain, including the codecs. So inadequate network capacity and the inability of the IP codecs to provide network layer reliability with the high MPX load is one major factor that can degrade the MPX signal performance. In addition, the codec’s quality of timing recovery and with analog MPX transport, the quality of the analog to digital processing chain can degrade the MPX signal quality. So a successful implementation will require all of these to fall in place.
RW: What are the benefits of distributing FM MPX over IP?
Parikh: MPX distribution provides several advantages. First, it enables engineers to located FM signal processing equipment, such as the audio processor, RDS generator and stereo generator, at the studio. Second, it saves on cap-ex associated with multisite distribution. Having a common set of equipment at the studio greatly simplifies the FM single-frequency network implementation.
RW: Why would anyone send an analog MPX signal along an IP network?
Parikh: Between the analog and digital MPX modes, digital MPX (AES192) is the preferred mode. The digital signal path provides better signal quality.
Analog MPX mode is selected when either the exciter does not have the support for an AES192 interface, or when the IP network capacity is not sufficient for the AES192 mode. For example, if the user is looking to transport the main audio channel and the RDBS subcarrier, it would require about 30% less data rate with analog MPX mode than with AES192.
RW: What are the factors that affect analog or digital MPXes on an IP network?
Parikh: Network impairments such as packet losses and jitter are probably the number one factor that can degrade the quality of the MPX signal. There can even be packet impairments with private or managed ISP connections that guarantee IP bandwidth, including complete loss of connection. More emphasis needs to be put on the codecs’ techniques for mitigating packet losses. The quality of timing recovery, which can be affected by network jitter, is another factor that can degrade the regenerated signal at the receiving codec.
RW: How can these be mitigated?
Parikh: The IP codecs distributing the MPX signal require purpose-built capabilities at both the network layer and signal level. At the network layer, the platform must provide packet protection techniques at the high MPX packet load. For example, techniques such as forward error correction to recreate lost packets, and Intraplex Dynamic Stream Splicing, which uses duplicate packet streams with time and network diversity to protect against both packet and connection losses.
At the signal layer, the codec’s DSP techniques must ensure that the fidelity of the analog MPX signal is maintained across the IP network, and that timing recovery is robust. Several options must be available for timing recovery. Besides packet-based recovery algorithms, in some use cases, it would be necessary to utilize common timing using GPS or PTP reference at both the encoder and decoder.
RW: How can a station’s/group’s IP network be optimized for an MPX?
Parikh: With IP networks becoming ubiquitous, there are many options with both private wireless links and managed ISP providers, but it is important that the users qualify the network connection for the load the encoder will generate. The best way to do this is to use the IP codec with the operational configuration to send packets and analyze the performance using an analytics application such as Intraplex LiveLook. The packet loss pattern, jitter and delay profile of the connection will help configure the stream’s network protection efficiently. For example, FEC protection is most efficient for isolated packet losses. With burst packet losses, time-delayed redundant streams are effective. The receive jitter buffer size setting is also critical: in most cases this can be set statically based on network profile. However, in some cases the codec’s ability to dynamically adapt the buffer will become important. Finally, the users should look for opportunities for redundant network connections, which will help with both packet loss and network loss protection.
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To see more show Q&As visit our NAB Show 2016 web page.
