GFS Projection and Sound Equipment

Like most community film societies, Godalming Film Society shows films by using an LCD projector connected to a DVD player.

Following a successful bid to the 'National Lottery Awards for All' we purchased a Sanyo PLC-XP100 projector that delivers 6500 ANSI lumens and a contrast ratio of 2000:1. (This is particularly valuable when setting up equipment as full blackout is not required to obtain a workable image.) It also offers a range of connections including DVI (HDMI connector), Composite and S-video. It supports up scaling HDMI output to 1080i. In addition it has excellent lens shift capabilities. The down side is the weight of the projector which is considerable (11.75 kg) and the need to insert and remove the lens at each screening to avoid damage when transporting it.

A long throw lens (LNS-T32) is used to project over a distance of 20m onto a 6m wide x 4.5m high roller projection screen. We have the advantage of raked seating and can comfortably accommodate an audience of 200. Our usual attendance is about 120.

The DVD player is a Denon 1930 offering a comprehensive range of controls and outputs including HDMI/ DVI and both analogue and digital sound.

For sound reproduction we have taken the same route as other film societies in opting for stereo sound through a good quality PA system (HK Audio LUCAS Performer). Whilst not taking advantage of the joys of surround sound it does mean that the equipment can be set up more quickly (and more safely as there are less cables!) and more easily controlled. We use a small sound desk (Peavey) to control the sound source and quality before and during screenings. With a target audience of 200 we have chosen a system that can deliver 1800 WRMS - roughly 1 W per film viewer which seems to be a useful guideline.

The Lip Sync Problem

Modern DVD players, Televisions and Digital Projectors use significant amounts of computer processing to extract and modify the digital data from the source (DVD) to produce high resolution images. Less processing is required to obtain high quality sound reproduction from the same source even when delivering surround sound from multiple speakers.

In particular, when using stereo sound from a DVD player while driving a high quality image from a digital projector (e.g. when composite or HDMI/ DVI signals are used), the sound will be available more quickly than the associated video image. Hence the source of the sound in the picture will appear to follow the sound. If the sound is associated with an action (hitting a drum for example) the sound will be heard before the action takes place. Words will be heard before lips move! These are examples of the 'lip sync problem' (lip synchronisation) and are very common when using high definition images.

The difference in video and sound processing complexity is not the sole source of Lip Sync problems. Poorly manufactured DVDs (such as pirate copies!) frequently exhibit the problem. Nor is the effect constant as the variation in sound and image processing times will change according to the amount of information that must be processed. More complex images require more time - the synchronisation problem will increase as images demand more processing.

Nevertheless, if a constant 'delay' can be built into the sound processing so that the image typically 'slightly' precedes the sound, this would effectively solve the problem. The human brain is more tolerant of images received ahead of associated noises as this is part of our normal environment in which light travels faster than sound! However, the extent of the required 'delay' is not obvious. It is likely to be less than a quarter of a second (250 milliseconds) and will need to be adjusted 'on the spot' to address the specific characteristics of the DVD being shown in the particular screening location.

The solution we have adopted comprises two elements:

1. A digital audio delay device
   Using the digital audio output (co-axial or optical connection) from the DVD player, this device introduces a delay into the digital sound 'channel' of up to 700 milliseconds. It does not convert the digital sound to analogue stereo sound. It is important that the device does not degrade the signal. Also, it is important that the delay is dynamically adjustable - preferably using a remote control - so that it can be changed while screening to control extreme occurrences of the problem.
   
   Product selected: The Felston DD740 - Audio Delay
   www.felston.com
   
   
2. A Digital to Analogue Converter (DAC)
   This component receives the digital signal from the 'delay' box and converts it into an analogue stereo signal that can be fed to the sound system. Once again it needs to be of a high quality so that no signal degradation is introduced. Interestingly, there are numbers of 'evaluations' that support the notion that a good quality external DAC can significantly enhance the quality of sound reproduction. DVD players contain an internal DAC which while adequate for everyday use is not necessarily the best!
   
   Product selected: The Beresford TC-7510 DAC
   www.homehifi.co.uk/TC-7510/about.html
   
   It is worth mentioning that the DVD player must be set to output digital sound in PCM (Pulse Code Modulation) format not Bitstream to operate this DAC.

Notes:

1. The evolving generation of better quality DVD players (Yamaha, Marantz etc.) do provide digital delay capabilities but these must be accessed via the menu (which will be displayed on the screen) and may only apply to the digital signal. Any changes to the delay while screening are not practical as they will be visible to the audience! Even with the advent of HDMI 1.3 which offers the possibility of lip sync management within the DVD player it is likely that adjustments will be 'screen visible'.
2. There is an alternative technology known as a Delay Line which is used in recording studio settings, is typically more expensive and is designed to be permanently (rack) installed.
3. The problem can be addressed using a laptop computer and appropriate software. It is possible to significantly reduce the amount of image processing required and to 'condition' the audio component. However, experience has shown that this approach is not sufficiently reliable due to the comparatively low quality of the integral DVD device in laptop computers and requires advanced computer skills!

Schematic

Technical details

Projector	Sanyo PLC -XP100 with long throw lens T32 ANSI Lumens: 3500 Contrast Ratio 900:1 www.ivojo.co.uk/sanyo-plc-xp100.htm	Connection DVI (HDMI) Composite S-Video
Sound System	HK Audio LUCAS Performer 1800 W RMS www.hkaudio.com	Subwoofer with integral amplifier Satellite speakers x 2 Speaker stands x 2
Sound desk / mixer	Peavey PV8
DVD Player	Denon DVD 1930 www.denon.co.uk
Projector Stand	Unicol: Base, column and platform
Audio delay device	Felston DD740 - Audio Delay www.felston.com	Connection Digital co-axial cable
Digital to Analogue Convertor	Beresford TC-7510 DAC www.homehifi.co.uk/TC-7510/about.html	Connection Digital co-axial cable

April 2008