Delays in TV subtitling remain an issue for viewers

05 November 2014

Ofcom today published its second report on the quality of live subtitles in UK TV programmes. The report shows that broadcasters are beginning to make improvements in some areas of subtitling. However, it finds that some issues - such as the delays between speech and the corresponding subtitle appearing on screen, known as latency - remain a problem.

Ofcom welcomes the fact that broadcasters are now making greater use of block subtitles in live programmes. These allow several words to appear at once as a single block and are easier and quicker for viewers to read than scrolling subtitles.

Last year, Ofcom required broadcasters to start measuring the quality of live TV subtitles. This work is intended to identify areas where broadcasters can improve subtitles to benefit viewers.

Today’s report is the second of four Ofcom is publishing over two years. Each report samples data from broadcasters on the quality of live TV subtitles - measuring their accuracy, speed and latency. The first report was published in April 2014.


The latest report finds that delays in subtitles reaching the screen continue to be a problem for viewers. For example, when programmes with live subtitles have frequent changes of scene - such as on Channel 4’s Gogglebox - viewers can find it difficult to know which scene the subtitles relate to.

Poor latency is one of the most frustrating aspects of live subtitling for TV viewers, who say that these delays result in a disjointed viewing experience.

Samples from BBC, ITV, Channel 4, Channel 5 and Sky programmes showed that the average (median) latency was 5.8 seconds (Figure 1), which exceeds the recommended guideline of a maximum 3 seconds delay. This is a 0.2 second increase in average latency since our first report.

Delays of up to 21 seconds were recorded, and only four samples out of the 72 measured had a median latency of less than 3 seconds.

Figure 1: Median latency of subtitles by broadcaster and genre Higher bars represent an inferior viewing experience


Source: Ofcom/broadcasters Note: not all broadcasters had all three live genres in the sample.


The accuracy of subtitling was rated as generally good by external reviewers auditing the data on behalf of Ofcom, although the quality varied. Ofcom’s external reviewers advise that a 98% accuracy score is seen as an acceptable standard (Figure 2).

Ofcom understands that broadcasters are now making greater efforts to provide subtitlers with advance information, such as running orders, pre-recorded film segments and scripts, on live programmes such as the news.

This may explain why more samples in this second reporting period showed very good to excellent (more than 99%) accuracy than the first set of data. However, the proportion of samples that failed to meet the 98% quality threshold rose from 20% to 25%.

Figure 2: Median accuracy of subtitles by broadcaster and genre Higher bars represent a better viewing experience


Source: Ofcom/broadcasters. Note: not all broadcasters had all three live genres in the sample.


In general, the programme subtitles sampled for the purposes of our second report met Ofcom’s current guidelines, which recommend a maximum speed of 160 - 180 words per minute.

The average speed of the subtitles (143 words per minute) was well below the recommended maximum, though there were some instances where speeds were unacceptably high. Speeds above the recommended maximum make it more difficult for viewers to read subtitles while watching the images on the screen.

Changes to help improve quality of subtitles

Ofcom’s first report on subtitles encouraged broadcasters to make greater use of block subtitles in live programmes. These allow several words to appear at once as a single block and are easier and quicker for viewers to read than scrolling subtitles.

Today’s report finds that broadcasters are now using more pre-prepared block subtitling in live programmes, which is welcomed by Ofcom.

However, pre-recorded subtitles being cued out too quickly caused some issues for viewers. Some of the programmes monitored showed incomprehensible speeds of up to 460 words per minute, far higher than Ofcom’s recommended 160 - 180 words per minute.Ofcom understands that software can be used to limit the speeds at which these subtitles are cued, so that it remains comprehensible to viewers. Ofcom expects broadcasters to take steps to resolve these teething problems.

By analysing subtitles broadcast over two years, Ofcom aims to help broadcasters identify opportunities to improve the experience for viewers who use subtitles. We also plan to use the data from the four sets of samples to review our guidelines on subtitling.

Ofcom expects to publish its third report on live subtitles in April 2015, drawing on samples taken from programmes broadcast in October and November 2014.



  1. Under the Communications Act 2003, Ofcom is required to have regard to the needs of persons with disabilities when carrying out its principal duty to further the interests of citizens and consumers under section 3 of the Act. Ofcom also has a specific duty under section 303 of the Act to provide guidance to broadcasters on how they should promote the understanding and enjoyment of their services by persons who are deaf or hard of hearing.
  2. Last year, Ofcom published a statement on the quality of subtitles. This required broadcasters to measure the accuracy, latency and speed of subtitles at six monthly intervals for a period of two years, with the results to be included in a series of reports published by Ofcom.
  3. The measurements for Ofcom’s second report were carried out by broadcasters or their contractors, and were checked for accuracy and consistency by a small team from the University of Roehampton for Ofcom. This exercise measured a total of 72 ten-minute clips from 72 programmes, which were analysed. These were from three genres (news, entertainment and chat shows) and broadcast on five channels (BBC, ITV, Channel 4, Channel 5 and Sky). In total, the analysis comprises twelve hours of live TV material including approximately 124,000 words and almost 19,000 subtitles.
  4. Ofcom has today also published its first report of 2014 on provision of access services by UK broadcasters. This first report (January to June 2014) shows that most broadcasters are making good progress towards comfortably exceeding their obligations for subtitling, signing and audio description. Where broadcasters are not meeting the requirements, Ofcom expects them to fulfil their obligations by the end of the year. We expect to publish the annual report in spring 2015.
  5. The Code on Television Access Services sets targets for the amount of TV subtitling, signing and audio description that broadcasters are required to provide. It also contains guidance about how access services should be presented.
  6. Audio description is a commentary woven around the soundtrack, exploiting pauses to explain on-screen action, describe characters, locations, costumes, body language and facial expressions to enhance meaning and enjoyment for blind or visually-impaired viewers.
  7. Sign language comprises the use of manual gestures, facial expression and body language to convey meaning. British Sign Language (BSL) is the main sign language in the UK.