Two Turntables and a Mobile Phone

Nicholas J. Bryan and Ge Wang

Center for Computer Research in Music and Acoustics (CCRMA)

Stanford University

660 Lomita Dr.

Stanford, California, USA

{njb, ge}@ccrma.stanford.edu

ABSTRACT

A novel method of digital scratching is presented as an al-

ternative to currently available digital hardware interfaces

and time-coded vinyl (TCV). Similar to TCV, the proposed

method leverages existing analog turntables as a physical in-

terface to manipulate the playback of digital audio. To do

so, however, an accelerometer/gyroscope–equipped smart

phone is ﬁrmly attached to a modiﬁed record, placed on a

turntable, and used to sense a performers movement, re-

sulting in a wireless sensing-based scratching method. The

accelerometer and gyroscope data is wirelessly transmitted

to a computer to manipulate the digital audio playback in

real-time. The method provides the beneﬁt of digital au-

dio and storage, requires minimal additional hardware, ac-

commodates familiar proprioceptive feedback, and allows

a single interface to control both digital and analog au-

dio. In addition, the proposed method provides numer-

ous additional beneﬁts including real-time graphical display,

multi-touch interaction, and untethered performance (e.g

“air-scratching”). Such a method turns a vinyl record into

an interactive surface and enhances traditional scratching

performance by aﬀording new and creative musical inter-

actions. Informal testing show this approach to be viable,

responsive, and robust.

Keywords

Digital scratching, mobile music, digital DJ, smartphone,

turntable, turntablism, record player, accelerometer, gyro-

scope, vinyl emulation software

1. INTRODUCTION

The performance practice of DJing has experienced aston-

ishing growth over the past three decades. Scratching, beat-

matching, beat juggling, mixing, and similar techniques can

be heard on the radio, in night clubs, and experimental mu-

sic contexts around the world. All such performance styles

can be traced back to the unique physical interaction and

expressive nature of a simple mechanical device–the ana-

log turntable. The simple physical control and inherent

proprioceptive feedback aﬀords the possibility of incredible

virtuosity and skill without hindering a beginner’s zeal.

With the advent of digital audio, however, great attention

has been focused on digital implementations of the turntable

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NIME’11, 30 May–1 June 2011, Oslo, Norway.

so as to leverage the many beneﬁts of digital storage and

playback. Such implementations typically fall within two

categories: methods leveraging existing analog turntables

with certain modiﬁcation and methods requiring alterna-

tive hardware mimicking the turntable’s control. Examples

of the prior include time-coded vinyl (TCV), while examples

of the latter include CDJs or similar interfaces [2, 1, 4, 3].

TCV uses a vinyl record encoded with time-code to detect

needle position, where as alternative hardware uses various

alternate sensing mechanisms. Both approaches have dis-

tinct advantages and disadvantages, largely dependent on

personal preference and performance style.

In many traditional DJ settings, time-coded vinyl meth-

ods have proven overwhelmingly popular. TCV methods

allow a single interface to control both analog and digital

audio and maintain the familiar and nimble scratching con-

trol of a traditional analog turntable. Disadvantages in-

clude wear and tear on the vinyl record, limited duration of

the TCV records, possible jumps in needle position during

a performance, and physical interference of the turntable

tone arm.

computer

(software + mixer)

mobile phone

local area network

analog turntables

Figure 1: Proposed DJ Setup. The system uses

turntables and sensor-equipped mobile phones, net-

worked with a host computer.

In this work, a novel method of digital scratching is pre-

sented as a viable alternative to currently available digital

hardware interfaces and TCV. The proposed method lever-

ages existing analog turntables as a physical interface to ma-

nipulate the playback of digital audio, but does not require a

time-coded record or any physical connection to a digital au-

dio playback device. An accelerometer/gyroscope–equipped

smart phone atop a modiﬁed record is used to wirelessly

transmit gesture data to a computer and manipulate the

digital audio playback accordingly as shown in Fig. 1. Us-

ing modern smart phones as a prototyping platform gives

similar beneﬁts as TCV, but provides numerous additional

beneﬁts including added visual display, multi-touch inter-

action, and untethered performance (e.g “air-scratching”),

turning a vinyl record into a familiar, but new tangible in-

terface [19]. Informal testing shows promising results, with

minimal latency and comparable feel when matched against

alternative approaches

2. RELATED WORK

Alternatives to commercially available digital DJ methods

include various form factors and implementations. Bill Ver-

plank references former students Keatly Halderman, Daniel

Lee, Steve Perella and Simon Reiﬀ replacing an existing

phonograph needle with a riding wheel equipped with an op-

tical shaft encoder to digitize the gesture control [30]. Hans

and others developed DJammer, an accelerometer equipped

MP3 player [27, 12, 13, 14]. In addition to untethered con-

trol of DJ gestures, DJammer presented the use of virtual

jam sessions to exchange and share music audio streams.

Sile O’Modhrain discusses the importance of haptic feed-

back for musical instrument design in [24] and emphasiz-

ing such importance, Beamish created the D’Groove haptic

turntable device in [5].

The use of wearable sensors for real-time music signal

processing is presented in [21], while an overview of design-

ing alternative tools for turntable music in the digital era

is presented in [22]. Villar et al. introduced the ColorDex

DJ System [31], using color as a mixing metaphor. Hansen

et al. have done extensive work on the acoustics and per-

formance of scratching [17, 15] as well as high level gesture

control [16, 18].

Multi-touch interfaces have also been used within many

musical and DJ applications including the popular Reactable

[20] and numerous commercially available DJ applications.

A gesture based mobile music game involving touch-screen

scratching was presented [11]. Most recently, Savage et al.

introduced a multi-modal mobile music mixer using mobile

phones with accelerometer for gesture control of Bluetooth

streaming audio [26].

Surveying such past academic work emphasizes the vari-

ous important diﬀerences in approach when designing new

turntable interfaces. Firstly, there is a distinction between

interfaces which are meant to enhance the turntable perfor-

mance experience while maintaining the traditional physical

interaction of manipulating motor movement versus inter-

faces which are meant to alter the interaction with equiv-

alent audio eﬀect. For better or worse, the goals between

the two approaches are notably diﬀerent. Secondly, there is

a diﬀerence between the DJ performance practices of mix-

ing and scratching. A large number of alternative interfaces

focus on mixing because of the latency and sensitivity re-

quirements. Scratching, described as the art of manipulat-

ing a vinyl record against a turntable needle, as well as the

related scribbling (rapid scratching) [17] requires accurate,

low-latency, highly-sensitivity sensing.

3. APPROACH

Within the various approaches found in recent and past

research, we present work towards the enhancement of the

turntable performance using existing analog turntable hard-

ware. In addition, we focus on digital scratching interaction.

As a general approach, we begin by leveraging the porta-

bility and computing power of modern mobile phones (or

smartphones), which have been shown to provide a highly

expressive compact form factor [32, 23]. Alternative sensors

such as a wirelessly enabled light sensors or similar have the

ability to oﬀer a more compact form factor for rotation sens-

ing, but do not provide numerous other advantages provided

by modern smartphones.

http://ccrma.stanford.edu/~njb/research/turntable/

3.1 Accelerometer and Gyroscope Sensing

Accelerometer and gyroscope–equipped smartphones, in par-

ticular, can be used to sense and wirelessly transmit ges-

tural control data. With proper processing, a three-axis

accelerometer and gyroscope can detect three-axis rotation

rate (pitch, roll, and yaw velocities) ideal for sensing motion

on a turntable. As a result, by ﬁrmly attaching a properly

equipped mobile phone atop a vinyl record, an existing ana-

log turntable can easily be modiﬁed into a digital scratch-

ing interface requiring no specialized sound card. Such a

method maintains a near equivalent sense of tactility and

results in a wireless sensing-based scratching method as seen

in Fig. 2. Further, the wireless sensing method does not

Figure 2: Wireless Sensor Record In Action. A pro-

totype record (combination of mobile phone, sticky

rubber, and plexiglass disc) resting on a standard,

commercially available turntable.

have any length limitation as found with TCV and advan-

tageously avoids physical interference of the turntable tone

arm.

For many situations, the capabilities of accelerometers

and gyroscopes are not ideal. The processing required is

non-trivial and demands careful attention. Small errors

in acceleration measurements propagate to larger errors in

velocity and position estimates, commonly referred to as

drift or bias. Gyroscopes, however, provide a complemen-

tary measure of orientation and can be used to improve

accelerometer measurements via complimentary ﬁlters, sta-

tistical ﬁlters (i.e. Kalman ﬁlters), or other methods collec-

tively referred to as sensor fusion algorithms. In addition,

physical constraints can be added to further improve esti-

mates such as limiting the axes of measurement. Serendipi-

tously, the motion of scratching gestures are limited around

a single axis and even more so, the motion is circular, di-

rectly relating centripetal force (provided by the performer)

to rotational velocity. As a result, the use of accelerometer

and gyroscope–equipped smartphones for precisely sensing

scratching gestures is surprisingly suitable.

3.2 Proposed Method

By processing the continuous data stream of the accelerom-

eter and gyroscope, the system can achieve a precise and ro-

bust measurement of instantaneous rotational velocity. This

allows us to robustly track both steady and variable rota-

tional velocity. Remarkably, this works well even for more

extreme changes in rotational velocities such as those pro-

duced by the physical gesture of scratching. By transmit-

ting this data over a low-latency wireless network, the phys-

ical gestures applied to the mobile phone can be mapped to

the playback position of an audio ﬁle or, more generally,

any other real-time audio parameter. This creates a viable

alternative to prior digital scratching methods.

4. INTERACTIONS

The proposed method enables a number of additional bene-

ﬁts and novel interactions, taking advantage of multi-touch

displays as well as the physicality and mobility of the mod-

ern mobile phone.

4.1 Visual Feedback

By placing a mobile phone on top of the moving vinyl record

and adding real-time “on-record”visual feedback with multi-

touch interaction, a simple vinyl record is transformed into

an interactive surface. This can aid a performer in numer-

ous contexts including cueing, scratching, and beat juggling.

The processing of cueing involves preparing one record to

mix in with another and involves matching tempo, musical

phrasing, or similar musical properties. Direct visual feed-

back on the record can help in this process, even suggesting

how to modify the speed controls of the analog turntable.

When performing scratching and beat juggling, DJs will

typically place a visual marker (e.g., tape, paint, Post-it,

etc.) to remember the playback point within a certain song

as described in [5]. Having on-record visual feedback can

directly aid in this process. Fig. 3 shows an example im-

plementation displaying the actual audio signal itself on

the moving record, visually displaying the current position

within a song. As the record moves, the visual display is up-

dated according to the exact position of the audio ﬁle play-

ing on the host computer with the window size or length

of the displayed audio controlled using multi-touch pinch-

to-zoom controls. As seen, the start of a percussive sound

Figure 3: On-Record Visual Feedback. An exam-

ple of on-record visual feedback displaying the time-

domain audio waveform via custom software.

is displayed indicating a possible physical location within

the record for scratching or beat juggling. More detailed

visualizations could display virtual paint markings, tape, or

Post-its for a familiar style indication or entirely diﬀerent

information and graphical user-interfaces. In addition, a

performer could use such visual feedback to select the “nee-

dle” position within a song or even switch between multiple

songs. As multi-touch technology advances, one could im-

age a multi-touch display covering the entire record surface.

4.2 Gesture Modiﬁcation

By using digitized gesture control, alternative gesture-to-

sound mappings are possible. As standard in digital DJing,

this can be found in the form of independent sensitivity,

pitch, and tempo control. More speciﬁcally, scratching ges-

ture can be ampliﬁed or dampened by scaling the transmit-

ted rotational rate accordingly.

Figure 4: Changing Tone Arm Position Aﬀects Ges-

ture Sensitivity. The changing tone arm position af-

fects scratching gestures as a function of line area.

As an alternative interpretation, sensitivity control can be

seen as controlling the tone arm position within a record.

Traditional analog turntables operate with constant veloc-

ity rotation, forcing audio material on the inner grooves of

a record to correspond to proportionally longer length sig-

nals for an equivalent angle as seen in Fig. 4. This causes

an exactly repeated scratching gesture to sound diﬀerently

depending on the tone arm position. This eﬀect can either

be replicated or removed depending on personal preference.

In addition, gesture sensing can be set relative to still or

constant motion. This allows the system to be used with or

without the turntable motor in action. The measured rota-

tion speed can simply be biased so still motion corresponds

to a playback rate of 1.0 instead of 0.0, allowing the per-

former to choose his or her preference. This is not possible

with traditional TCV, which requires active rotation.

Finally, as presented in [25, 8, 9] and others, such ges-

tural control can also be used for active listening, allowing

the general public of listeners and inexperienced users inter-

act with the music generation and manipulation, not just

trained musicians.

4.3 Untethered Scratching

While serving the purpose of enhancing traditional scratch-

ing gestures tethered to an existing analog turntable, the

presented approach aﬀords alternative interactions that can

lead to new forms of expression. In particular, untethered

or “air” scratching can be performed by simply lifting the

mobile phone-equipped record oﬀ the turntable as no phys-

ical sensor connections are required. As discussed in [27]

and [26], untethered interaction frees the performer to move

about and even interact directly with the audience. Such

ability poses numerous interesting questions regarding im-

provisation techniques and other musical devices. Involving

audience participation during a live scratch performance,

for example, is an appealing direction of study.

5. IMPLEMENTATION

For implementation, custom software for both the sensing

smartphone and host computer was needed with minimal

custom hardware. More detailed hardware and software

implementation issues are discussed in §5.1 and §5.2 respec-

tively.

Figure 5: Untethered Scratching Interaction. “Air-

Scratching” is possible with or without a physically

attached record.

5.1 Hardware

For hardware, a single mobile phone, piece of sticky rubber,

and plexiglass disc were used for each wirelessly enabled

record. Fourth generation iPod Touch or iPhone 4 devices

were found to work well. Both devices include a three-axis

accelerometer and three-axis gyroscope with a maximum

sample rate of 100 Hz as well as a multi-touch display and

wireless networking capabilities. The iPod Touch is phys-

phone

record

sticky rubber

Figure 6: Wireless Sensor Enabled Record. A mo-

bile phone attached to a modiﬁed record via sticky

rubber.

ically thinner than the iPhone 4 and found to be slightly

preferable. In order to ﬁrmly attach the device to a vinyl

record, sticky rubber is placed in between the record and

mobile device as shown in Fig. 6. Commercially available

rubber mats (used to hold mobile phones against a car dash-

board) were used and found to be suﬃciently sticky.

Various plexiglass discs were used in place of a vinyl

record. By varying the weight and size of record, a per-

former can customize the drag or friction between the record

and slipmat to suit their needs. A collage of prototype im-

ages is found in Fig. 7, showing a single record with phone

and rubber, collection of various discs, and the complete DJ

setup.

5.2 Software

Software implementation came in two forms: software on

the mobile phone and host computer software. The mo-

bile phone software was written within Apple’s iOS SDK

along with portions of the Mobile Music Toolkit [7], osc-

pack [6], and the Synthesis Toolkit [10]. The iOS Core-

Motion framework gives an excellent mechanism to stream

processed accelerometer and gyroscope data by an unspeci-

ﬁed sensor fusion algorithm (most likely complementary or

Kalman ﬁltering), directly providing three-axis rotational

rates of pitch, roll, and yaw. The yaw rate is then wirelessly

transmitted using Open Sound Control on top of UDP sock-

ets. Track information and other non-real-time information

can be sent reliably over TCP sockets.

Figure 7: Prototype Hardware. (Upper Left)

Phone, plexiglass disc, and sticky rubber. (Upper

Right) Various sized and weighted discs to accom-

modate a performer’s sense of tactility. (Lower)

Prototype setup.

To adequately implement the visual feedback of the cur-

rently playing audio stream as discussed in §4.1, additional

information including the position within the currently play-

ing audio ﬁle must be sent from the host computer back to

the mobile phone to update the display. If the visual display

is not updated by the host computer, the two devices will

drift from one another causing the audio and visuals to be-

come out-of-sync. Such eﬀect is confusing to the performer

and is greatly undesirable.

Host software employs the Jules’ Utility Class Exten-

sions (JUCE) [29] providing a cross-platform framework

with numerous tools ready for audio application develop-

ment. A traditional DJ software model is taken, allowing

two simultaneous audio streams. The host program receives

the transmitted rotational rate and manipulates the audio

stream by resampling. Linear interpolation was initially

used for prototyping, with improvements found when using

higher-order polynomial interpolation [28]. An image of the

developed software is shown in Fig. 8.

Figure 8: Custom Prototype DJ Software. The de-

veloped DJ software required to receive and pro-

cessing the mobile phone sensor data and manipu-

late audio accordingly.

6. DISCUSSION AND EVALUATION

As discussed earlier, there are various advantages and disad-

vantages when comparing diﬀerent digital scratching meth-

ods. In general, however, it is diﬃcult to objectively evalu-

ate new methods and compare against past work. Just as a

violinist gets a custom to the feel and sound of their instru-

ment, a DJ will learn the subtleties of a given scratching

method and can be averse to change [5]. Informal perfor-

mance testing, nevertheless, showed promising results with

minimal perceived latency between input gesture data and

output audio playback.

General measures of evaluation included precision and re-

sponsiveness as well as stability. Rapid physical gestures

were seen to be very responsive and have precise corre-

sponding audio eﬀect. Repeated physical gestures were also

found to have a consistent sounding eﬀect over long perfor-

mance times. Further testing with professional level DJs,

however, is needed for a more complete evaluation. For

a video demonstration of the system in action please see

http://ccrma.stanford.edu/~njb/research/turntable/.

The one-way network latency time between a given phone

and host computer was measured to be on average 3-5 ms

and compares favorably with professional audio recording

equipment. When comparing the maximum humanly-possible

scratch rate (10-20 turns per second [17]), the 100 Hz sam-

ple rate of the accelerometer and gyroscope appears suit-

able. The perceived eﬀect of accelerometer and gyroscope

latency, however, is diﬃcult to measure and dependent on

the sensor ﬁltering method used, requiring further study

and user evaluation.

7. CONCLUSIONS

A straightforward and surprisingly eﬀective method of digi-

tal scratching is presented. The proposed method leverages

existing analog turntables as a physical interface and takes

advantage of the capabilities of modern sensor–equipped

smartphones, resulting in a genuinely physical, wireless sensing-

based scratching method. Beneﬁts include digital audio and

storage, minimal additional hardware, familiar propriocep-

tive feedback, and a single interface to control both digital

and analog audio. Further beneﬁts include visual display,

gesture modiﬁcation, and the possibility of interactions un-

tethered from the turntable. Testing and evaluation show

this approach to be viable and promising.

8. ACKNOWLEDGMENTS

This work was enabled by National Science Foundation Cre-

ative IT grant No. IIS-0855758 as well as the funding from

the School of Humanities and Sciences, Stanford University.

Additional thanks to Professor Jonathan S. Abel for valu-

able conversation regarding the tone arm control. Finally,

a thank you to the anonymous reviewers for valuable feed-

back regarding the application of active listening and track

switching, among other observations.

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