Scream: A DCI Mark Lapslie Investigation by Nigel McCrery Read Free Book Online Page A

at Lapslie over the top of his glasses. ‘Meanwhile, that sound file you sent me. How did you get hold of it?’
    ‘It was sent to me,’ Lapslie said. ‘I have no idea who by.’
    ‘An anonymous gift. Beware of strings attached.’
    ‘So what can you tell me about it?’
    Burrows stood up. ‘Take a walk with me,’ he said. ‘We’ll head down to the audio lab.’
    The building, one of many scattered across the site, took the form of a central spine corridor from which spur corridors radiated at right angles. Burrows led Lapslie down the main corridor and then diverted off, having to type a four-figure code into a keypad before the door would unlock for him. Doorways to either side gave access to white-painted laboratories lined with wooden benches and computer screens. Burrows led the way into one such laboratory which looked to Lapslie little different from the others.
    ‘Sara,’ he said to the blonde-haired girl who was sitting at a bench wearing a white coat. ‘Do you have that sound file up?’
    ‘Just working on it now,’ she said, glancing from Burrows to Lapslie and back. She looked barely older than Lapslie’s sons.
    ‘Mark, this is Sara Hawkins. Sara, this is DCI Lapslie. Can you give him an update on where we are?’
    ‘Sure.’ She thought for a moment, then indicated the computer sitting on the bench in front of her. It was flanked by two large speakers, and the screen indicated what looked to Lapslie like images of two long, white mountain ranges reflected in lakes, one above the other. ‘This is a pictorial representation of the amplitude of the sound file over time,’ she started. ‘As you can see, it’s a stereo signal – two channels, slightly different from one another – so it was recorded with two microphones. The sampling rate is 44.1 kilohertz, with 16-bit resolution for each channel, which means that the sound is sampled 44,100 times per second. That’s CD-level quality.’
    She smiled at Lapslie’s blank expression. ‘It’s okay. Look, any sound that’s been recorded and then translated into digital format exists as a series of numbers. The numbers represent the characteristics of the sound, like volume and frequency – takena number of times per second. If you think of the original sound as a smooth curve then the digital representation is a series of steps which try to match the curve. The more steps you have per second, the smoother the resulting profile looks. So, 44,100 samples per second, with 16 bits per channel, and two channels, will result in a bit-rate of 1,411,200 bits per second. That’s almost perfect sound quality. A sample recorded at a rate of, say, 320,000 bits per second is listenable. Even 190,000 bits per second is more than acceptable for most purposes. By the time you get down to 128,000 bits per second then you can hear a noticeable degradation in quality, something like a radio broadcast, and 64,000 bits per second is more like listening to something over a phone than anything else. Clear?’
    ‘I have no idea what you’re talking about,’ Lapslie said in a heartfelt voice.
    She smiled. ‘Don’t worry. If you take one thing away from this, take away the fact that the sound file was recorded to preserve as much information from the source as possible, given the constraints of data storage.’ She paused, and fixed him with a piercing gaze. ‘Whoever recorded it wanted it to sound good. They
cared
about the sound quality.’
    Lapslie thought for a moment. ‘You’ve indicated that there’s a trade-off at work there – sound quality versus size of file. Would it be possible to record it in a way that preserved as much of the sound as was technically possible?’
    ‘Oh yes,’ she said. ‘But that would take up a huge amount of hard disk space, assuming it was being stored on a computer.’
    ‘So why would this person compromise on the sound quality?’
    ‘So they could store more sound files,’ she said brightly. Her face fell. ‘Oh.