diff --git a/frontend/src/Waveform.tsx b/frontend/src/Waveform.tsx
index a3d327a..75e6b36 100644
--- a/frontend/src/Waveform.tsx
+++ b/frontend/src/Waveform.tsx
@@ -2,11 +2,7 @@ import { useEffect, useState, useRef, MouseEvent } from 'react';
 import { Waveform as WaveformOverview } from './Waveform/Overview';
 import { Thumbnails } from './Waveform/Thumbnails';
 import { Canvas as WaveformCanvas } from './Waveform/Canvas';
-import {
-  secsToCanvasX,
-  canvasXToFrame,
-  mouseEventToCanvasX,
-} from './Waveform/Helpers';
+import { canvasXToFrame, mouseEventToCanvasX } from './Waveform/Helpers';
 
 type Props = {
   audioContext: AudioContext;
@@ -65,6 +61,25 @@ export const Waveform: React.FC<Props> = ({ audioContext }: Props) => {
   // TODO: error handling
   const videoID = new URLSearchParams(window.location.search).get('video_id');
 
+  // helpers
+
+  // secsToCanvasX returns the logical x coordinate for a given position
+  // marker. It is null if the marker is outside of the current viewport.
+  const secsToCanvasX = (secs: number): number | null => {
+    if (mediaSet == null) {
+      return null;
+    }
+
+    const frame = secs * mediaSet.audio.sampleRate;
+    if (frame < zoomSettings.startFrame || frame > zoomSettings.endFrame) {
+      return null;
+    }
+
+    const logicalPixelsPerFrame =
+      CanvasLogicalWidth / (zoomSettings.endFrame - zoomSettings.startFrame);
+    return (frame - zoomSettings.startFrame) * logicalPixelsPerFrame;
+  };
+
   // effects
 
   // setup player on page load:
@@ -180,11 +195,10 @@ export const Waveform: React.FC<Props> = ({ audioContext }: Props) => {
         return;
       }
 
-      const x = secsToCanvasX(
-        currentTime,
-        mediaSet.audio.sampleRate,
-        mediaSet.audio.frames
-      );
+      const x = secsToCanvasX(currentTime);
+      if (x == null) {
+        return;
+      }
 
       ctx.strokeStyle = 'red';
       ctx.beginPath();
@@ -279,7 +293,6 @@ export const Waveform: React.FC<Props> = ({ audioContext }: Props) => {
   };
 
   const handleSelectionChange = (selection: Selection) => {
-    console.log('in handleSelectionChange handler');
     if (mediaSet == null) {
       return;
     }
diff --git a/frontend/src/Waveform/Helpers.tsx b/frontend/src/Waveform/Helpers.tsx
index 3512e3b..2226b9c 100644
--- a/frontend/src/Waveform/Helpers.tsx
+++ b/frontend/src/Waveform/Helpers.tsx
@@ -5,7 +5,6 @@ import { MouseEvent } from 'react';
 export const mouseEventToCanvasX = (
   evt: MouseEvent<HTMLCanvasElement>
 ): number => {
-  // TODO: use offsetX/offsetY?
   const rect = evt.currentTarget.getBoundingClientRect();
   const elementX = evt.clientX - rect.left;
   return (elementX * CanvasLogicalWidth) / rect.width;
@@ -15,13 +14,3 @@ export const mouseEventToCanvasX = (
 export const canvasXToFrame = (x: number, numFrames: number): number => {
   return Math.floor((x / CanvasLogicalWidth) * numFrames);
 };
-
-// TODO: add tests
-export const secsToCanvasX = (
-  secs: number,
-  sampleRate: number,
-  numFrames: number
-): number => {
-  const duration = numFrames / sampleRate;
-  return Math.floor(CanvasLogicalWidth * (secs / duration));
-};