(Based on 
High-end, 4-turntable/deck solution for professional DJs.
The Rane SL4 for Serato Scratch Live is the first standalone DJ interface to offer two USB ports, superior sound with 96kHz sample rate and 24-bit digital audio processing and galvanic isolation to eliminate computer noise and interference.
The SL4 connects four vinyl turntables or CD players to your computer and the mixer of your choice. Using special control vinyl, CDs or MIDI controllers, you can scratch and mix files from your Mac or Windows computer's CD or hard drive. This intuitive system replicates the traditional DJing experience for "real feel" usability.
Teamed with low latency ASIO and Core Audio drivers, the SL 4 can also be used as a high-grade studio production tool—all of this in one powerful package, masterfully crafted by Rane.
Included software: Scratch Live
Scratch Live supports all these file formats: Fixed and Variable Bit Rate MP3, AIFF, AAC, ALAC, WAV, OGG Vorbis, CD Audio and direct from the mixer. Record from your mixer's Aux output and immediately scratch as if it was on vinyl. Record direct from vinyl or CD and save as a 24-bit / 48kHz or 96kHz stereo file.
Easily import your playlists and iTunes library. You can drag from Explorer (Windows) or the Finder (Mac) into the Scratch Live library. The crate and subcrate system helps organize your library and sets. Hot-swap external drives with fast automatic library merging and retention of crates. Browse your music collection by album, artist, BPM, or genre, with optional album art display. Intuitive search function can be applied to any field to find things quickly. Edit ID3 tags with full Unicode support.
Pre-processing of MP3 files is not required except to detect corrupt files or use the auto-BPM and auto-gain functions. Auto-BPM detects tempo information for tracks without it, saving it with each file. A tap tempo button allows you to enter your own. Auto-gain lets you set a target gain on any or all songs in your library. You can also manually adjust individual gain, remembered in each song file.
The Virtual Deck shows everything about the speed and position of a track. As the vinyl rotates, so does the line on the label. The circular progress bar around the edge is a visual representation of the position within the song. The time and remaining time are displayed in minutes and seconds. The turntable speed as a percentage of pitch shift is on the left, and the pitched BPM is on the right.
Different operating modes can be assigned to each Virtual Deck:
Absolute Mode—exact reproduction of record movement, needle drop through tracks – just like normal vinyl. The beginning of the track is mapped to the start of the record.
Relative Mode—skip-free scratching, jump to cue points, and pitch bend buttons combined with turntable control. This mode observes the relative forward and backward movement of the record, but does not regard the position within the record.
Internal Mode—mix with just your laptop using features modeled on DJ CD players. Rewind, Bend down, Play / pause reverse, Play / pause forward, Bend up, Fast forward.
Auto mode—play a sequence of tracks automatically.
With Key Lock on, the key or pitch of the song stays locked at what it would be if the track was playing at normal speed, regardless of the platter speed of the turntable or CD player. Key Lock has scratch detection, so it automatically turns off when scratching for a natural scratching sound.
Variable zoom waveforms centered around current position display horizontally or vertically. Waveforms are colored by frequency so you can distinguish instruments visually. Full track overview lets you quickly find where the beat drops, find breakdowns and choruses, and jump to any part of the song. The tempo display assists with rapid tempo matching of tracks. When the two tracks are matched to the same tempo, the peaks will line up. Beat display emphasizes the transients in tracks—these are matched up when the two tracks are beat matched. Visual markers can be set for up to 5 cue points per song and triggered from the keyboard. Save up to 9 loops per track—these loops are saved in the file, and will be present when you reload. Censor expletives on the fly with an instant quick-reverse and catch-up with no loss of timing or pitch.
The software's MIDI control can interface USB controllers with Scratch Live. DJ controller support will vary. Check the latest notes at serato.com for supported controllers. Built-in help and tips in multiple languages are accessible anytime. There is even a low power warning when running off a laptop battery.
Control Vinyl Spares
Continuous control signal on vinyl allows smooth accurate scratching. Scratch Live has the lowest latency vinyl emulation system on the market. It has advanced vinyl groove and stylus modeling algorithm for a sound indistinguishable from the real thing. The Innovative positioning system allows fast-and-accurate needle dropping. The record has thin marks every minute, and a thicker mark every 5 minutes (played at 33 RPM). These marks assist you when using needle dropping to move through the track.
The Vinyl Scroll track allows you to use your turntable or CD to scroll through your crate and pick the next song without touching your laptop.
Four copies of black vinyl are included with the SL 4. Side A: 10 minutes plus Vinyl Scroll track. Side B: 15 minutes.
Based on 
