Two Turntables and a Mobile Phone

Nicholas J. Bryan and Ge WangCenter for Computer Research in Music and Acoustics (CCRMA)

Stanford University660 Lomita Dr.

Stanford, California, USA{njb, ge}@ccrma.stanford.edu

ABSTRACTA novel method of digital scratching is presented as an al-ternative to currently available digital hardware interfacesand time-coded vinyl (TCV). Similar to TCV, the proposedmethod leverages existing analog turntables as a physical in-terface to manipulate the playback of digital audio. To doso, however, an accelerometer/gyroscope–equipped smartphone is firmly attached to a modified record, placed on aturntable, and used to sense a performers movement, re-sulting in a wireless sensing-based scratching method. Theaccelerometer and gyroscope data is wirelessly transmittedto a computer to manipulate the digital audio playback inreal-time. The method provides the benefit of digital au-dio and storage, requires minimal additional hardware, ac-commodates familiar proprioceptive feedback, and allowsa 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 intoan interactive surface and enhances traditional scratchingperformance by affording new and creative musical inter-actions. Informal testing show this approach to be viable,responsive, and robust.

KeywordsDigital scratching, mobile music, digital DJ, smartphone,turntable, turntablism, record player, accelerometer, gyro-scope, vinyl emulation software

1. INTRODUCTIONThe performance practice of DJing has experienced aston-ishing growth over the past three decades. Scratching, beat-matching, beat juggling, mixing, and similar techniques canbe heard on the radio, in night clubs, and experimental mu-sic contexts around the world. All such performance stylescan be traced back to the unique physical interaction andexpressive nature of a simple mechanical device–the ana-log turntable. The simple physical control and inherentproprioceptive feedback affords the possibility of incrediblevirtuosity and skill without hindering a beginner’s zeal.

With the advent of digital audio, however, great attentionhas been focused on digital implementations of the turntable

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so as to leverage the many benefits of digital storage andplayback. Such implementations typically fall within twocategories: methods leveraging existing analog turntableswith certain modification and methods requiring alterna-tive hardware mimicking the turntable’s control. Examplesof the prior include time-coded vinyl (TCV), while examplesof the latter include CDJs or similar interfaces [2, 1, 4, 3].TCV uses a vinyl record encoded with time-code to detectneedle position, where as alternative hardware uses variousalternate sensing mechanisms. Both approaches have dis-tinct advantages and disadvantages, largely dependent onpersonal preference and performance style.

In many traditional DJ settings, time-coded vinyl meth-ods have proven overwhelmingly popular. TCV methodsallow a single interface to control both analog and digitalaudio 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 ofthe TCV records, possible jumps in needle position duringa performance, and physical interference of the turntabletone arm.

computer(software + mixer)

mobile phone

mobile phone

local area network

analog turntables

Figure 1: Proposed DJ Setup. The system usesturntables 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 digitalhardware 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 atime-coded record or any physical connection to a digital au-dio playback device. An accelerometer/gyroscope–equippedsmart phone atop a modified record is used to wirelesslytransmit gesture data to a computer and manipulate thedigital audio playback accordingly as shown in Fig. 1. Us-ing modern smart phones as a prototyping platform givessimilar benefits as TCV, but provides numerous additionalbenefits 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, withminimal latency and comparable feel when matched againstalternative approaches1.

2. RELATED WORKAlternatives to commercially available digital DJ methodsinclude various form factors and implementations. Bill Ver-plank references former students Keatly Halderman, DanielLee, Steve Perella and Simon Reiff replacing an existingphonograph needle with a riding wheel equipped with an op-tical shaft encoder to digitize the gesture control [30]. Hansand others developed DJammer, an accelerometer equippedMP3 player [27, 12, 13, 14]. In addition to untethered con-trol of DJ gestures, DJammer presented the use of virtualjam 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 hapticturntable device in [5].

The use of wearable sensors for real-time music signalprocessing is presented in [21], while an overview of design-ing alternative tools for turntable music in the digital erais presented in [22]. Villar et al. introduced the ColorDexDJ System [31], using color as a mixing metaphor. Hansenet al. have done extensive work on the acoustics and per-formance of scratching [17, 15] as well as high level gesturecontrol [16, 18].

Multi-touch interfaces have also been used within manymusical and DJ applications including the popular Reactable[20] and numerous commercially available DJ applications.A gesture based mobile music game involving touch-screenscratching was presented [11]. Most recently, Savage et al.introduced a multi-modal mobile music mixer using mobilephones with accelerometer for gesture control of Bluetoothstreaming audio [26].

Surveying such past academic work emphasizes the vari-ous important differences in approach when designing newturntable interfaces. Firstly, there is a distinction betweeninterfaces which are meant to enhance the turntable perfor-mance experience while maintaining the traditional physicalinteraction 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 betweenthe two approaches are notably different. Secondly, there isa difference between the DJ performance practices of mix-ing and scratching. A large number of alternative interfacesfocus 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 therelated scribbling (rapid scratching) [17] requires accurate,low-latency, highly-sensitivity sensing.

3. APPROACHWithin the various approaches found in recent and pastresearch, we present work towards the enhancement of theturntable 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 (orsmartphones), which have been shown to provide a highlyexpressive compact form factor [32, 23]. Alternative sensorssuch as a wirelessly enabled light sensors or similar have theability to offer a more compact form factor for rotation sens-ing, but do not provide numerous other advantages providedby modern smartphones.1http://ccrma.stanford.edu/~njb/research/turntable/

3.1 Accelerometer and Gyroscope SensingAccelerometer and gyroscope–equipped smartphones, in par-ticular, can be used to sense and wirelessly transmit ges-tural control data. With proper processing, a three-axisaccelerometer and gyroscope can detect three-axis rotationrate (pitch, roll, and yaw velocities) ideal for sensing motionon a turntable. As a result, by firmly attaching a properlyequipped 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 amethod maintains a near equivalent sense of tactility andresults in a wireless sensing-based scratching method as seenin Fig. 2. Further, the wireless sensing method does not

Figure 2: Wireless Sensor Record In Action. A pro-totype record (combination of mobile phone, stickyrubber, 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 tonearm.

For many situations, the capabilities of accelerometersand gyroscopes are not ideal. The processing required isnon-trivial and demands careful attention. Small errorsin acceleration measurements propagate to larger errors invelocity and position estimates, commonly referred to asdrift or bias. Gyroscopes, however, provide a complemen-tary measure of orientation and can be used to improveaccelerometer 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 arounda 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 accelerometerand gyroscope–equipped smartphones for precisely sensingscratching gestures is surprisingly suitable.

3.2 Proposed MethodBy 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. Thisallows us to robustly track both steady and variable rota-tional velocity. Remarkably, this works well even for moreextreme 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 viablealternative to prior digital scratching methods.

4. INTERACTIONSThe proposed method enables a number of additional bene-fits and novel interactions, taking advantage of multi-touchdisplays as well as the physicality and mobility of the mod-ern mobile phone.

4.1 Visual FeedbackBy placing a mobile phone on top of the moving vinyl recordand adding real-time“on-record”visual feedback with multi-touch interaction, a simple vinyl record is transformed intoan 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 tomix in with another and involves matching tempo, musicalphrasing, or similar musical properties. Direct visual feed-back on the record can help in this process, even suggestinghow to modify the speed controls of the analog turntable.

When performing scratching and beat juggling, DJs willtypically place a visual marker (e.g., tape, paint, Post-it,etc.) to remember the playback point within a certain songas described in [5]. Having on-record visual feedback candirectly aid in this process. Fig. 3 shows an example im-plementation displaying the actual audio signal itself onthe moving record, visually displaying the current positionwithin 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 lengthof 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 withinthe record for scratching or beat juggling. More detailedvisualizations could display virtual paint markings, tape, orPost-its for a familiar style indication or entirely differentinformation and graphical user-interfaces. In addition, aperformer could use such visual feedback to select the “nee-dle” position within a song or even switch between multiplesongs. As multi-touch technology advances, one could im-age a multi-touch display covering the entire record surface.

4.2 Gesture ModificationBy 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 beseen 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 ofa record to correspond to proportionally longer length sig-nals for an equivalent angle as seen in Fig. 4. This causesan exactly repeated scratching gesture to sound differentlydepending on the tone arm position. This effect can eitherbe replicated or removed depending on personal preference.

In addition, gesture sensing can be set relative to still orconstant motion. This allows the system to be used with orwithout the turntable motor in action. The measured rota-tion speed can simply be biased so still motion correspondsto a playback rate of 1.0 instead of 0.0, allowing the per-former to choose his or her preference. This is not possiblewith 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, allowingthe general public of listeners and inexperienced users inter-act with the music generation and manipulation, not justtrained musicians.

4.3 Untethered ScratchingWhile serving the purpose of enhancing traditional scratch-ing gestures tethered to an existing analog turntable, thepresented approach affords alternative interactions that canlead to new forms of expression. In particular, untetheredor “air” scratching can be performed by simply lifting themobile 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 moveabout and even interact directly with the audience. Suchability poses numerous interesting questions regarding im-provisation techniques and other musical devices. Involvingaudience participation during a live scratch performance,for example, is an appealing direction of study.

5. IMPLEMENTATIONFor implementation, custom software for both the sensingsmartphone and host computer was needed with minimalcustom hardware. More detailed hardware and softwareimplementation issues are discussed in §5.1 and §5.2 respec-tively.

Figure 5: Untethered Scratching Interaction. “Air-Scratching” is possible with or without a physicallyattached record.

5.1 HardwareFor hardware, a single mobile phone, piece of sticky rubber,and plexiglass disc were used for each wirelessly enabledrecord. Fourth generation iPod Touch or iPhone 4 deviceswere found to work well. Both devices include a three-axisaccelerometer and three-axis gyroscope with a maximumsample rate of 100 Hz as well as a multi-touch display andwireless 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 stickyrubber.

ically thinner than the iPhone 4 and found to be slightlypreferable. In order to firmly attach the device to a vinylrecord, sticky rubber is placed in between the record andmobile device as shown in Fig. 6. Commercially availablerubber 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 vinylrecord. By varying the weight and size of record, a per-former can customize the drag or friction between the recordand slipmat to suit their needs. A collage of prototype im-ages is found in Fig. 7, showing a single record with phoneand rubber, collection of various discs, and the complete DJsetup.

5.2 SoftwareSoftware implementation came in two forms: software onthe mobile phone and host computer software. The mo-bile phone software was written within Apple’s iOS SDKalong 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 streamprocessed accelerometer and gyroscope data by an unspeci-fied sensor fusion algorithm (most likely complementary orKalman filtering), directly providing three-axis rotationalrates of pitch, roll, and yaw. The yaw rate is then wirelesslytransmitted using Open Sound Control on top of UDP sock-ets. Track information and other non-real-time informationcan be sent reliably over TCP sockets.

Figure 7: Prototype Hardware. (Upper Left)Phone, plexiglass disc, and sticky rubber. (UpperRight) 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, additionalinformation including the position within the currently play-ing audio file must be sent from the host computer back tothe mobile phone to update the display. If the visual displayis not updated by the host computer, the two devices willdrift from one another causing the audio and visuals to be-come out-of-sync. Such effect is confusing to the performerand is greatly undesirable.

Host software employs the Jules’ Utility Class Exten-sions (JUCE) [29] providing a cross-platform frameworkwith numerous tools ready for audio application develop-ment. A traditional DJ software model is taken, allowingtwo simultaneous audio streams. The host program receivesthe transmitted rotational rate and manipulates the audiostream by resampling. Linear interpolation was initiallyused for prototyping, with improvements found when usinghigher-order polynomial interpolation [28]. An image of thedeveloped 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 EVALUATIONAs 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 aviolinist gets a custom to the feel and sound of their instru-ment, a DJ will learn the subtleties of a given scratchingmethod and can be averse to change [5]. Informal perfor-mance testing, nevertheless, showed promising results withminimal perceived latency between input gesture data andoutput audio playback.

General measures of evaluation included precision and re-sponsiveness as well as stability. Rapid physical gestureswere seen to be very responsive and have precise corre-sponding audio effect. Repeated physical gestures were alsofound 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. Fora video demonstration of the system in action please seehttp://ccrma.stanford.edu/~njb/research/turntable/.

The one-way network latency time between a given phoneand host computer was measured to be on average 3-5 msand compares favorably with professional audio recordingequipment. When comparing the maximum humanly-possiblescratch 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 gyroscopelatency, however, is difficult to measure and dependent onthe sensor filtering method used, requiring further studyand user evaluation.

7. CONCLUSIONSA straightforward and surprisingly effective method of digi-tal scratching is presented. The proposed method leveragesexisting analog turntables as a physical interface and takesadvantage of the capabilities of modern sensor–equippedsmartphones, resulting in a genuinely physical, wireless sensing-based scratching method. Benefits include digital audio andstorage, minimal additional hardware, familiar propriocep-tive feedback, and a single interface to control both digitaland analog audio. Further benefits include visual display,gesture modification, and the possibility of interactions un-tethered from the turntable. Testing and evaluation showthis approach to be viable and promising.

8. ACKNOWLEDGMENTSThis work was enabled by National Science Foundation Cre-ative IT grant No. IIS-0855758 as well as the funding fromthe 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 trackswitching, among other observations.

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