Home > Articles > Cisco Network Technology > IP Communications/VoIP > Designing IP-Based Video Conferencing Systems: Dealing with Lip Synchronization

Designing IP-Based Video Conferencing Systems: Dealing with Lip Synchronization

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Dec 7, 2007.

Chapter Description

Two issues typically complicate the process of decoding audio and video streams and allowing them to play with perfect synchronization. This chapter covers the process of realigning the audio and video streams at the receiver to achieve perfect lip synchronization.

This chapter covers the following topics:

  • Understanding lip sync skew
  • Lip sync approaches
  • Understanding the sender side
  • Understanding the receive side
  • Real-time Transport Protocol
  • Correlating time bases using RTCP

Chapter 3, "Fundamentals of Video Compression," went into detail about how audio and video streams are encoded and decoded in a video conferencing system. However, the last processing step in the end-to-end chain involves ensuring that the decoded audio and video streams play with perfect synchronization. This chapter focuses on audio and video; however, video conferencing systems can synchronize any type of media to any other type of media, including sequences of still images or 3D animation. Two issues complicate the process of achieving synchronization:

  • Real-time Transport Protocol (RTP)-based video conferencing systems separate audio and video into different RTP streams on the network.
  • Video conferencing systems also typically have separate processing pipelines for audio and video within the sender and receiver endpoints.

This chapter covers the process of realigning those streams at the receiver.

Understanding Lip Sync Skew

Lip sync is the general term for audio/video synchronization, and literally refers to the fact that visual lip movements of a speaker must match the sound of the spoken words. If the video and audio displayed at the receiving endpoint are not in sync, the misalignment between audio and video is referred to as skew. Without a mechanism to ensure lip sync, audio often plays ahead of video, because the latencies involved in processing and sending video frames are greater than the latencies for audio.

Human Perceptions

User-perceived objection to unsynchronized media streams varies with the amount of skew—for instance, a misalignment of audio and video of less than 20 milliseconds (ms) is considered imperceptible. As the skew approaches 50 ms, some viewers will begin to notice the audio/video mismatch but will be unable to determine whether video is leading or lagging audio. As the skew increases, viewers detect that video and audio are out of sync and can also determine whether video is leading or lagging audio. At this point, the video/audio offset distracts users from the video conference. When the skew approaches one second, the video signal provides no benefit—viewers will ignore the video and focus on the audio.

Human sensitivity to skew differs greatly from person to person. For the same audio/video skew, one person might be able to detect that one stream is clearly leading another stream, whereas another person might not be able to detect any skew at all.

A research paper published by the IEEE reveals that most viewers are more sensitive to audio/video misalignment when audio plays before the corresponding video, because hearing the spoken word before seeing the lips move is more "unnatural" to a viewer (Blakowski and Steinmetz 1996).

Sensitivity to skew is also determined by the frame rate and resolution: Viewers are more sensitive to skew when watching higher video resolution or higher frame rate.

Report IS-191 issued by the Advanced Television Systems Committee (ATSC) recommends guidelines for maximum skew tolerances for broadcast systems to achieve acceptable quality. The guidelines model the end-to-end path by assuming that a single encoder at the distribution center receives both audio and video streams, digitizes the streams, assigns time stamps, encodes the streams, and then sends the encoded data over a network to a receiver. The guidelines specify that on the sending side, at the input to the encoder, the audio should not lead the video by more than 15 ms and should not lag the video by more than 45 ms. This possible lead or lag might arise from uncertainty in the latencies through the digitizing/capture hardware and occurs before the encoder assigns time stamps to the digitized media streams.

At the receiving side, the receiver plays the audio and video streams according to time stamps assigned by the encoder. But again, there is an uncertainty in the latency of each stream through the playout hardware. The guidelines stipulate that for each stream, this uncertainty should not exceed ±15 ms; this tolerance is an absolute tolerance that applies to each stream. Based on these guidelines, two requirements emerge for acceptable lip sync tolerance:

  • Criterion for leading audio—In the worst-case-permitted scenario, audio leads video at the input to the encoder by 15 ms. The receiver plays the audio stream too far ahead by 15 ms while playing the video stream too far behind by 15 ms. As a result, the maximum amount by which audio may lead video at the presentation device of the receiver is 15 ms + 15 ms + 15 ms = 45 ms.
  • Criterion for lagging audio—In the worst-case-permitted scenario, audio lags video at the input to the encoder by 45 ms. The receiver plays the audio stream too far behind by 15 ms while playing the video stream too far ahead by 15 ms. As a result, the maximum amount by which audio may lag video at the presentation device of the receiver is 45 ms + 15 ms + 15 ms = 75 ms.

Measuring Skew

Audio/video skew is measured on the output device at presentation time. The output device is also called the presentation device. The definition of presentation time depends on the output device:

  • For video displays, the presentation time of a frame in a video sequence is the moment that the image flashes on the screen.
  • For audio devices, the presentation time for a sample of audio is the moment that the endpoint speakers emit the audio sample.

The presentation times of the audio and video streams on the output devices must match the capture times at the input devices. These input devices (camera, microphone) are also called capture devices. The method of determining the capture time depends on the media:

  • For a video camera, the capture time for a video frame is the moment that the charge-coupled device (CCD) in the camera captures the image.
  • For a microphone, the capture time for a sample of audio is the moment that the microphone transducer records the sample.

For each type of media, the entire path from capture device on the sender to presentation device on the receiver is called the end-to-end path.

A lip sync mechanism must ensure that the skew at the presentation device on the receiver is as close as possible to zero. In other words, the relationship between audio and video at presentation time, on the presentation device, must match the relationship between audio and video at capture time, on the capture device, even in the presence of numerous delays in the entire end-to-end path, which might differ between video and audio.

Figure 7-1 provides another way of looking at media synchronization. This diagram shows the timing of multiple streams playing out the presentation devices of a receiver, without synchronization.

Figure 7-1

Figure 7-1 Receive-Side Stream Skews Without Synchronization

Each stream could be a video or audio stream. The gray marker in each stream corresponds to the same time at the sender, referenced to a clock on the sender that is common to all inputs. This common reference clock is also referred to as a common reference timebase. For these streams to play in a synchronized manner, the gray markers must line up; that is, the samples at the gray markers must emerge from the playout devices simultaneously. The goal is to add delay to the streams that play "too early" (streams 1, 2, and 4) so that they play in sync with stream 3, which is the stream that arrives "too late."

Delay Accumulation

Skew between audio and video might accumulate over time for either the video or audio path. Each stage of the video conferencing path injects delay, and these delays fall under three main categories:

  • Delays at the transmitter—The capture, encoding, and packetization delay of the endpoint hardware devices
  • Delays in the network—The network delay, including gateways and transcoders
  • Delays at the receiver—The input buffer delay, the decoder delay, and the playout delay on the endpoint hardware devices

However, most of these delays are unknown and difficult to measure and change over time. This means that the mechanism for achieving lip sync should not attempt to measure and account for each individual delay in the end-to-end media path. Instead, the mechanism must work in the presence of variable, unknown path delays.

Most video conferencing equipment transmits audio and video over a network using RTP, which multiplexes audio and video into separate network streams. This method is in contrast to the format for DVDs, which multiplex the audio and video streams into a single stream called an MPEG2 program stream. Because the audio and video streams of a video conference remain separated through the network from endpoint to endpoint, each stream might experience different network delays.

Figure 7-2 shows how differing delays in the end-to-end audio and video paths can accumulate over time, causing the skew between audio and video to increase at each stage of the media path.

Figure 7-2

Figure 7-2 Audio and Video Skew Accumulation

The first graph at the upper left shows the original relationship between video and audio. The graph represents audio as a sequence of packets forming a continuous stream. Each audio packet spans a duration of time corresponding to the audio data it contains. In contrast, the graph represents video as a sequence of frames, where each frame exists for a single instant of time. The figure shows a scenario in which the skew between audio and video increases at three stages of the end-to-end path from sender to receiver: after the sender-side delays, after the network delays, and after the receiver-side delays. To understand how delays creep into each stage, it is necessary to look at how each stage processes data, starting with the network path.

Delays in the Network Path

A lip sync solution must work in the presence of many delays in the end-to-end path, both in the endpoints themselves and in the network. Figure 7-3 shows the sources of delay in the network between the sender and the receiver. The network-related elements consist of routers, switches, and the WAN.

Figure 7-3

Figure 7-3 End-to-End Delays in a Video Conferencing System

The network also hosts other elements that may process media streams: conference bridges, transraters, and transcoders. These devices might add considerable delay to one or both streams and might cause the network delay for one stream to be significantly greater than the network delay for the other stream.

Bridges combine video/audio streams from multiple endpoints to facilitate a multipoint conference. The process of mixing or combining streams imposes an end-to-end delay.

Transraters re-encode a video stream into a lower bit rate to send the bitstream through a lower-bandwidth network or to a lower-bandwidth endpoint. Transraters typically apply only to video streams.

Transcoders may exist in the network to change the codec type and may apply to either audio or video streams. Figure 7-3 shows a transcoder that translates from G.711 to G.728. A video conferencing network configuration might require transcoders for two reasons:

  • To reduce the bit rateFigure 7-3 shows a scenario in which an audio transcoder converts a high-bandwidth audio stream into a low-bandwidth stream. In this case, the high-bandwidth G.711 stream arrives at the transcoder on a high-bandwidth LAN, and the bridge must transcode the audio stream into a lower-bandwidth G.728 version suitable for a low-bandwidth WAN. When a bridge uses a transcoder for the sole purpose of changing the bit rate, it is still called a transcoder, even if the end effect is that of a transrater.
  • To bridge two endpoints with different codec capabilities—One example for audio is the process of converting from an H.320-centric G.729 codec to an H.323-centric G.723 codec. An example for video is the process of converting from an H.323-centric H.263 codec to an H.320-centric H.261 codec.

Delays for audio and video on some segments of the network might differ due to different quality of service (QoS) levels. Figure 7-3 shows a router configured with QoS to provide lower latency for audio than for video. This difference in quality might be continuous or might arise only when the router suffers heavier-than-normal network congestion.

The congestion level of routers might cause the delays for either audio or video to fluctuate over time. In the figure, router X temporarily experiences a heavy load at time T, causing it to momentarily increase the delay of packets through its queue.

In addition to these short-term events, the long-term, steady-state network path taken by either stream might abruptly change as a result of a change in the dynamic IP routing. Any change in IP routing results in new steady-state end-to-end delays.

2. Lip Sync Approaches | Next Section

Cisco Press Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from Cisco Press and its family of brands. I can unsubscribe at any time.

Overview

Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about Cisco Press products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites; develop new products and services; conduct educational research; and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@ciscopress.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security

Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children

This site is not directed to children under the age of 13.

Marketing

Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out

Users can always make an informed choice as to whether they should proceed with certain services offered by Cisco Press. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.ciscopress.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links

This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020