Download recording of lecture here (20MB MP3)

Lecture Report

Dr Emmett opened the lecture by summarising the audio-video synchronisation challenges encountered when putting together a television programme. It is better to correct synchronisation problems as they occur in the broadcasting chain than to attempt to correct them all immediately prior to transmission, as the former practice greatly simplifies video editing. With this achieved, attention turns to keeping audio synchronised during broadcast transmission and reception. This is particularly important for human speech: humans are exquisitely sensitive to lip sync. We develop this facility almost as soon as we can see, and the psychological need for lip movement to be attached to speech is so great that each Dalek must display a light that pulses in sync with speech, in order to bond dialogue to a particular character.

A number of techniques were employed in the days of purely analogue transmission to ensure that audio and video were kept in sync. It was not unusual for a programme’s video signal to be relayed via satellite and its audio via telephone, and a compensating audio delay had to be inserted to offset uplink and downlink delays. An example of this was used in ITN in the early 1980s. An in-band masked ‘bong’ was timed to follow any video cut in the programme by exactly one second. It was possible then for engineers to adjust the audio delay manually to maintain sync, even where this varied during the programme. Similar timestamps must still be maintained in digital systems, although this facility is now generally accommodated within the channel code.

It is increasingly common for audio and video to be streamed by piggy-backing on a packet-based protocol and transmitting via existing IT infrastructure. This works as long as there is sufficient bandwidth. Otherwise, heavy-duty interleaving is required to compensate for dropped packets, which increases transmission delay, and the chances of sync loss and system failure. As with real piggy-backs, the heavier the payload, the slower the system, and the greater the likelihood of collapse.

Consider what the word ’standard’ means: this is where problems are compounded. The word has two distinct meanings. It can refer to an outgoing or obsolescent paradigm (such as ’standard definition’), or to standard-bearing in its original sense — at the technological vanguard. We frequently encounter problems when it is necessary to choose between a plenitude of competing standards of different ages, some of which have yet to be adopted, and many of which should not. Standards are necessary only when the current best practice is unclear, so clues for choosing ‘good standards’ were suggested. A good standard must be fit for purpose, timely, and robustly defined: if the plug fits, the signal should work. There are also caveats, because not all standards are intended to be friendly (DRM systems were cited as an example), and even de facto standards undergo sudden and complete changes. Finally, although a standard needs to be owned by a company or committee to avoid obsolescence, it should contain no element for revenue generation.

The emergence of competing delivery standards in broadcasting has brought the synchronisation problem into the home. Many digital multichannel audio transport layers can be conveyed over S/PDIF channel code using IEC 61937 (Dolby Digital; DTS; linear PCM), and a home cinema amplifier may typically accommodate sixty connectors and a dozen multichannel formats. As for the picture, high-definition video formats such as 720p and 1080p co-exist with conventional 625-line 4:3 and 16:9 broadcasts. There are a number of video interconnection formats with different costs, advantages, and limitations. Any of four digital video broadcasting standards are in use in different regions throughout the world, encompassing several standard frame rates. Meanwhile, individual consumer products are designed for world markets, and are simultaneously compatible with many of these standards. In fact, UK broadcasters have been unable to rely on viewers possessing ’standard’ receiving equipment since 625-line broadcasts began in the 1960s.

Now that it can take half a day for a professional engineer to set up a new television, it is quite likely that a set-top box in a typical home may be configured to down-convert 720p video to standard definition, and transmit this signal over RGB SCART to a plasma television, which will then up-convert it to 1080p. Audio-video synchronisation is then at the mercy of equipment manufacturers.

Dr Emmett summarised his lecture with advice from Antoine de Saint-Exupéry: ‘No design is finished until the last superfluous item has been removed.’

Report by Ben Supper

We’ve managed to squeeze in a visit to British Grove Studios that was built by Mark Knopfler as “a monument to past and future technology”. It has two studios. Alongside modern technology, the equipment includes two old EMI mixing consoles: a very rare tube desk from the 1960s like the ones used by George Martin and The Beatles and a later console on which the album Band on the Run was actually recorded. The large console in Studio One is a custom-made Neve 88R, while the console in Studio Two is an API Legacy.

In February 2009, British Grove Studio was the winner of the Music Producers Guild Award for ‘Best Studio’.

The visit is for AES members only and numbers limited. Please email/phone Heather Lane (or use the contact form here, select “membership and general enquiries” option) to reserve a space and confirm availability.

[Note: due to commercial pressures, there is a small, but finite chance, that this visit might get cancelled at the last minute - sorry, but we think the opportunity to visit this great facility is worth the chance! For this reason, it is imperative we get your contact details so, should the worst happen, we can let you know]

Technical Visit to British Grove Studios, London

We’ve managed to squeeze in a visit to British Grove Studios that was built by Mark Knopfler as “a monument to past and future technology”. It has two studios. Alongside modern technology, the equipment includes two old EMI mixing consoles: a very rare tube desk from the 1960s like the ones used by George Martin and The Beatles and a later console on which the album Band on the Run was actually recorded. The large console in Studio One is a custom-made Neve 88R, while the console in Studio Two is an API Legacy.

In February 2009, British Grove Studio was the winner of the Music Producers Guild Award for ‘Best Studio’.

The visit is for AES members only and numbers limited. Please email/phone Heather Lane (or use the contact form here, select “membership and general enquiries” option) to reserve a space and confirm availability.

[Note: due to commercial pressures, there is a small, but finite chance, that this visit might get cancelled at the last minute - sorry, but we think the opportunity to visit this great facility is worth the chance! For this reason, it is imperative we get your contact details so, should the worst happen, we can let you know]

Location: Royal Academy of Engineering, London
Description: Lecture by Dr Lauren Stewart, Goldsmiths University, London
Start Time: 19:00
Date: Thursday 10th June 2010

The ability to make sense of musical sound has been observed in every culture since the beginning of recorded history. In early infancy, it allows us to respond to the sing-song interactions from a primary caregiver and to engage in musical play. In later life it shapes our social and cultural identities and modulates our affective and emotional states. But a few percent of the population fail to develop the ability to make sense of or engage with music.  The study of disordered musical development sets in sharp relief the perceptual and cognitive abilities which most of us take for granted and give us a unique chance to investigate how musical perceptual ability develops, from the level of the gene to the brain development and the emergence of a complex and fundamental human behaviour.

Dr Stewart is Senior Lecturer and director of a new MSc course:

Music, Mind and Brain at Goldsmiths, University of London

Lauren originally studied Physiological Sciences at Balliol College Oxford, but transferred from bodies to brains with an MSc in Neuroscience and doctoral and postdoctoral training at the Institute of Cognitive Neuroscience, the Wellcome Department of Imaging Neuroscience (both UCL) and Harvard Medical School.

Her current research interests ranges from studying those with congenital amusia who have an inability to make sense of musical sound to studying the acquisition of perceptual, cognitive and motor skills in trained musicians.

The twice-yearly AES Convention is the most significant event in the AES calendar, bringing together audio professionals around the world for the leading technical conference and trade show for all sectors of the professional audio industry. The Call for Papers (pdf) deadline for the conference is 18th December, so time is running out if you have findings to present. For further details and updates on the London AES Convention, please visit the Convention website at http://www.aes.org/events/128/.

An excellent Tutorial by Neville Thiele can be found here

Neville Thiele’s name is known to anyone who has ever taken an interest in the practical design of moving coil loudspeakers, through the Thiele-Small parameters that bear his name and that of Richard Small. In 1961 he wrote a seminal paper on the design of vented (reflex) loudspeakers that – although it was largely ignored for 10 years until reproduced in the AES Journal – is now acknowledged as initiating the filter parameter based approach to loudspeaker analysis and synthesis which today is routinely used by the audio industry at large. In recognition of this, in 1994 he was awarded the AES Silver Medal.

In this interview-based lecture, Neville Thiele will talk about what led up to this breakthrough and its significance to the speaker design process. He will then give three short presentations on loudspeaker-related topics: filter-assisted bass alignments and novel crossover approaches; driver ageing effects; and driver impedance correction in crossover networks. Questions will then be invited from the audience.

The lecture is open to all, 18:30 for 19:00 start, and will be held at the Royal Academy of Engineering in London.

Lecture Report

John Vanderkooy presented research into methods to improve loudspeaker measurements made in non-anechoic rooms. The lecture began with a discussion of the motivation for the research:

• Not everyone has access to an anechoic chamber
• Anechoic chambers may not be effective below 100Hz due to inadequate LF absorption
• Low frequency calibration of anechoic chambers may be ineffective
• Low frequency noise from air conditioning, industry and the environment can easily contaminate the measurements.

Impulse response measurements made in an echoic room or an imperfect anechoic chamber will have reflections that contaminate the results and will also often have significant levels of added noise. John presented measurements from a 110mm driver in a small sealed cabinet to illustrate the algorithm developed to overcome these limitations.

The algorithm comprises the following steps:

1)    Measure an impulse response, typically 5–6ms of which is reflection-free following the initial response of the loudspeaker, and obtain the frequency response..
2)    Apply a minimum phase filter to the impulse data such that the frequency response becomes flat to DC and, optionally, a high-pass filter with a corner frequency significantly above that of the loudspeaker.
3)    Truncate the impulse response such that all room reflections are removed.  The resulting frequency response will have high-pass characteristic at a higher corner frequency.
4)    Apply an inverse filter to that of step 2.

Now the impulse response has the low frequency persistent decaying oscillation  extending cleanly beyond the first reflection arrival time.

There are several impulse response windowing methods and filter types that can be used. John explained that a rectangular window introduces ripples into the frequency response, while other types cause data to be lost towards the end of the truncated impulse response.

Methods of shortening the impulse response  discussed were the Backman method and the Fincham method. The Backman method of flattening the frequency response to DC causes the impulse response to have a very long but zero-valued tail, making it suitable for truncation. The Fincham method, which raises the apparent corner frequency of the loudspeaker’s LF roll-off, shortens the impulse response, again allowing truncation to be applied without significant loss of data in the tail. As originally described, the Fincham method seemed to apply the step 2 filter to the test signal, which results (when the inverse filter is applied) in increased contamination of the acoustic measurement by low frequency noise. This can be avoided by applying the step 2 filter to the measured impulse response instead, and apparently this was the method actually employed.

Results obtained from a mid-size test speaker measured in a reverberant space were presented to show that reflections contaminate the measured frequency response if not windowed out. If they are windowed out conventionally, however, the frequency response at low frequencies is inaccurate because the impulse response is truncated prematurely. Whereas if the impulse response is processed using a 5ms rectangular window and Fincham filtering the result is a much more accurate frequency response below 200Hz.

Design of the Fincham filter requires knowledge of the loudspeaker’s bass alignment, which can be obtained either from analysis of its impedance versus frequency behaviour or from a near-field acoustic measurement. Accuracy of the frequency response obtained from the processed impulse response is not too dependent on the alignment parameters used..

John explained that the resulting low frequency response has a strong imprint of the model applied but argued that the result is still useful because we have good knowledge of the behaviour of loudspeakers at low frequencies. He also demonstrated that cabinet diffraction does not compromise the method, whereas it does provide difficulties for Prony Method modelling of the impulse response because diffraction cannot be modelled as an exponentially decaying oscillation.

John concluded the lecture by showing that conventionally gated impulse responses have validity at mid and high frequencies, so that obtaining the low frequency response using the method described gives a final measurement result which is in large part free of imperfections caused by room reflections across the entire audible frequency range. John ended the lecture by encouraging all present to try this methodology for themselves.

Report by Matthew Neighbour and Keith Howard