Home > Articles > Networking > Voice/IP Communications

  • Print
  • + Share This
This chapter is from the book

Lip Sync Approaches

Video conferencing endpoints generally take two approaches to achieve lip sync:

  • Poor Man's lip sync—This method assumes that delays in the end-to-end media paths are known and constant. It relies on packet arrival times for synchronization.
  • Common Reference lip sync—This method assumes that delays in the end-to-end media paths are not easily known and might vary. It relies on a common reference timebase for both audio and video streams.

Poor Man's Lip Sync

The simplest incarnation of a lip sync algorithm is known as Poor Man's lip sync. In this method, the receiver uses one criterion to synchronize audio and video: Packets of audio and video that arrive simultaneously at the network interface of the receiver are considered to be synchronized to each other. This approach is fundamentally flawed because delays in the end-to-end path vary both in space (at different points of the path) and time (fluctuations in delay from one moment to the next). In addition, trying to measure, predict, and compensate for these variable delays in the end-to-end path is a futile effort.

When using Poor Man's lip sync, the conferencing system must make several assumptions about the sender, receiver, and network infrastructure:

  • Sender—For Poor Man's lip sync, the sender generally assumes that the network delay to the receiver is the same for both audio and video streams. However, scenarios might arise in which the delays differ. For instance, a transcoder might be present in the audio path but not the video path. Or, the network might assign a higher QoS to one path, resulting in lower delay for that stream.
  • Receiver—When operating with Poor Man's lip sync, the receiver must derive a relationship between the time stamps of the two streams by observing the relationship between the packet arrival times and timestamps for each individual stream, and then using that information to derive a relationship between the packet timestamps of the two streams. However, the receiver might have difficulty deriving an accurate relationship, because packet arrival times vary because of arrival-time jitter.
  • Network infrastructure—Poor Man's lip sync makes the following invalid assumptions:
    • - The average network delay remains constant over the long term.
    • - The instantaneous network delay remains constant over the short term.

When you are using Poor Man's lip sync, if the sender cannot compensate for network delays, specialized video conferencing network infrastructure might be necessary between the two endpoints. This infrastructure readjusts the synchronization of audio and video streams by adding delay to one stream or another.

An unfortunate byproduct of Poor Man's lip sync is that it often results in the sender or the network infrastructure delaying one or more streams to attempt synchronization at the receiver. However, for maximum flexibility, delays should be introduced only at the receivers, which leads to one of the corollaries of robust lip sync:

  • Only the receiver should delay media streams to achieve lip sync.

This corollary is necessary for two reasons:

  • If the end-to-end audio delay is already significant, the receiver might prefer to avoid adding more delay to the audio stream and forego lip sync. Instead, the receiver might want to go without audio and video synchronization to maintain the lowest audio end-to-end delay for the best interaction between conference participants. The end user makes this decision via the user interface (UI) of the receiver endpoint. Therefore, other entities on the network should not overrule this decision.
  • The process of delaying one stream to achieve lip sync should be left to the receiver, because the receiver can take into account its own internal delays in each media path at the same time that it delays one stream or the other to synchronize media. For instance, if audio arrives at the receiver ahead of video, normally the receiver must delay the audio stream to achieve lip sync with the video. However, an input buffer in the receiver might already provide some or all of this delay.

Common Reference Lip Sync

The goal of lip sync is to preserve the relationship between audio and video in the presence of fluctuating end-to-end delays in both the network and the endpoints themselves. Therefore, the most important restriction to keep in mind when discussing lip sync for video conferencing is the following:

  • Video conferencing systems cannot accurately measure or predict all delays in the end-to-end path for either the audio or video stream.

This restriction leads to the most important corollary of lip sync:

  • A method of lip synchronization must not use a mechanism that attempts to measure and compensate for individual delays in the end-to-end path.

The second corollary addresses the method that systems should use to achieve lip sync:

  • A method of lip synchronization must use timestamps that can be correlated to a common timebase.

Before considering a robust method of synchronization using a common reference, it is necessary to cover the data path inside the sender and receiver of a video conference.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

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


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about 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.


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.


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


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


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


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.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. 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.informit.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.


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